Douglas Adams (pictured) described his 1987 novel, Dirk Gently's Holistic Detective Agency as a "thumping good detective-ghost-horror-who dunnit-time travel-romantic-musical-comedy-epic", and while this may seem on the surface to be something of a flippant over exaggeration from Adams, typical of his wry wit, incredibly it is a synopsis that is entirely accurate.

The story is indeed a detective murder mystery, but it's far from conventional: there are ghosts, time travel is involved, and it's Douglas Adams, so of course it's dripping with his deadpan humour. However, what really sets this book apart is the genuine drama behind the jokes that could be classed as horror. It's even possible to describe the novel as a musical (if it's possible for a book to be a musical?). Though, if we're honest there isn't an enormous amount of romance... we'll let him off that one.

One thing is for sure: Dirk Gently is certainly a novel of epic proportions. But the question is, how can a book that wholeheartedly embraces all these genres actually work? To find the answer you have to look at the thing as a whole. In short, you might say that you have to take a holistic approach...

Gently is a deeply mysterious, self-styled detective, who believes in "the fundamental interconnectedness of all things". His ad in the phone book immodestly assures potential clients that he will solve "the whole crime" and find "the whole person". He's created the myth that he's psychic, mystic, telepathic, and a "psychosassic vampire bat". But when our protagonist, Richard MacDuff, meets him he's in the middle of several phone calls to an array of angry old cat ladies who want to know why Dirk has run up enormous bills holidaying in the Bahamas, while apparently searching for their missing cats. Dirk assurances them that these seemingly outrageous expenses all make perfect sense at a quantum level.

MacDuff knew Dirk when they were at the University of Cambridge together, although Richard hesitates to call him a friend. The idea of Dirk actually having a companion is quoted as seeming: "not so much unlikely, more a sort of mismatching of concepts, like the idea of the Suez Crisis popping out for a bun".

He quickly finds himself dragged back into Dirk's bizarre world when he's invited to Cambridge by his former tutor - the distinctly batty Professor Chronotis - to attend a formal dinner to celebrate the poet Samuel Taylor Coleridge. This kicks off a remarkable chain of events that find MacDuff the prime suspect in the murder of his boss, Gordon Way. There's only one man who can save him, and as it turns out the entire human race... Dirk Gently.

It was Douglas Adams's first novel away from the Hitchhiker's Guide To The Galaxy series (aside from The Meaning Of Liff, co-written with John Lloyd). Dirk Gently has its roots in several Doctor Who episodes, which Douglas Adams wrote during his time working as a script editor on the 17th series, in particular, the now-legendary Shada.

For those who aren't familiar, 1979's Shada is an ill-fated six-part Doctor Who serial that had been entirely scripted by Douglas Adams. The ambitious series was only partially filmed before a strike at the BBC halted all production, never to be resumed, and so becoming something of a legend amongst sci-fi fans.

Over the years Shada has been re-worked, novelised, animated, released on VHS with narrated sections, remade into an audio drama, and very nearly buried in soft peat for three months and recycled as firelighters, until it was finally patched together into a definitive version for both a Blu-ray release and a broadcast on BBC America. It debuted on 19th July 2017, a mere thirty-seven years, nine months and five days after the start of filming.

Shada and Dirk Gently share a common main character: Professor Chronotis, an old don at the University of Cambridge with an extraordinary secret. One of the greatest scenes in the book is Richard's visit to his old rooms, where they encounter a seemingly supernatural presence. It's hard to describe without giving away the joke as the comedy is all in the reveal, but it's a masterfully crafted moment that really forces a rethink of how comedy and drama can, and perhaps should, be merged.

The real drama stems from the murdered Gordon Way, who lives on after his death in ghostly torment. In various chapters we join him as he comes to the realisation that he's dead, and there's not a lot he can do about it.

Musings on death, and the terrifying reality of what it might actually be like to become a ghost are explored in some of Douglas Adams's darkest and most poignant writing. The horror is just enough to give Dirk Gently an edge, without becoming gratuitous. This more serious tone was something Adams managed to pull off neatly; he'd done so before with the surprisingly earnest romance between Arthur Dent and Fenchurch in the fourth Hitchhiker novel, So Long And Thanks For All The Fish. He had an ability to take comedy to a unique and sincere place. There's a moment in Dirk Gently where we're swept away with Richard, as he revels in the sheer wonderment of classical music. It's an utterly beautiful moment, a sort of modern soliloquy of that which Shakespeare, or indeed Coleridge, would be proud.

It's Coleridge that's of particular importance here. This novel is at its core a love letter to the arts, but the main source of inspiration are Samuel Taylor Coleridge's poems Kubla Khan and The Rime of the Ancient Mariner, which are integral to the plot. Yet these intellectual references manage to sit elegantly next to daft gags about sofas, Duran Duran posters, missing cats and naming Beatles songs - not to mention one of Adams's greatest and perhaps strangest characters, The Electric Monk.

What is an Electric Monk? Well, an Electric Monk is the ultimate labour-saving device: "Dishwashers washed tedious dishes for you, thus saving you the bother of washing them yourself, video recorders watched tedious television for you, thus saving you the bother of looking at it yourself; Electric Monks believed things for you, thus saving you what was becoming an increasingly onerous task, that of believing all the things the world expected you to believe."

And of course, an Electric Monk always rides a trusty steed. After all: "Things look more sincere on a horse".

Being careful not to spoil anything, it's worth stating that the specific mechanics of the ending are still debated, and famously Douglas Adams himself revealed on his own internet message boards that even he didn't understand some of the finer details - he gatecrashed a thread where fans were discussing it to admit:

"Ahem. All I can say is that it was as clear as day to me when I wrote it and now I can't figure it out myself. Sorry about that. I'm actually thinking about it at the moment as I've been re-reading the book in preparation for doing a screenplay. I've got a little bit of sorting out to do..."

A remark that was typical of his humility, but also showcased his love of technology. He had fostered a way to chat to his fans directly, long before Twitter had been invented.

It's true that the book is so intricately plotted that it makes your head spin trying to piece it all together. As soon as you finish it, you feel a strong desire to go straight back to the beginning to look for all the tiny clues that you missed the first time. There are so many little tip-offs you might call them 'dead giveaways', and they're such a joy when you find them. It's multi-layered humour, but more than that, each seemingly random joke, every strange occurrence or surreal aside that you initially put down to Adams's eccentric style, makes perfect sense in retrospect and has to be there to serve a later development. It's not just a delight to re-read, but perhaps even better the second time round.

Sadly, the screenplay that Adams referred to in his forum post is something that has yet to materialise on screen. It was inspired by a stage adaptation of the novel, Dirk, by James Gross and Arvind David, which ran at the Oxford Playhouse in 1997 featuring Rory Kinnear in the lead role: Douglas Adams was in attendance one evening and adored it.

David told Entertainment Weekly: "They say you should never meet your heroes. That's rubbish advice. Always meet your heroes. When we adapted [the first book for] the play, we changed the plot. I was absolutely terrified when Douglas turned up because I thought he would be upset because we had changed his plot. I said to him very timidly afterward, 'Is that okay, that we changed it?' And he went, 'Oh, you fixed it. Perfect.'"

After Douglas's untimely death in 2001 (aged just 49) the play went international, being staged in both Australia (2001) and California (2006). He had been living in Los Angeles, where he was focused on developing a Hitchhiker's Guide feature film, eventually realised by Disney in 2005.

However, a number of Dirk Gently television adaptations have been produced.

A version for BBC Four came first, running from 2010 to 2012, although for only four episodes. Starring Stephen Mangan as Dirk and Darren Boyd as Richard, Dirk Gently saw Mangan as a great Dirk: it was clear he'd put his heart and soul into the role, and he made no secret about how "bitterly upset" he was when the BBC axed the series. It was a great shame as the show captured the humour of the novels perfectly, whilst added a few further additions - particularly amusing was Dirk's lack of a whiteboard for brainstorming ideas, instead he scribbled them directly onto a white wall in his office, freshly painted for each new case, making sure that he always had a pot of paint and a roller to hand.

BBC America took a crack at adapting Dirk Gently in 2016, in conjunction with Netflix. Dirk Gently's Holistic Detective Agency starred Samuel Barnett as Dirk and Elijah Wood as Todd, a 'new' sidekick, although Richard in all but name. Fully reimagined in a truly modern style, the big-budget series - Dirk Gently for a Stranger Things audience - became a cult hit and resulted in an outcry from its own legion of fans when it too was axed in 2018.

However, neither television versions followed the plot of Adams's original novel, or its sequel, The Long Dark Tea-Time Of The Soul (published in 1988).

As a brief aside, it's worth mentioning a number of Dirk Gently shorts. Filmed for a 1992 episode of arts documentary strand The South Bank Show focused on Adams, they starred Michael Bywater. A friend of Douglas's whom he had admitted Dirk was in part based on, he acted out a scene from the book. It's noteworthy as this is the pure Dirk Gently that Adams had envisioned.

A rather charming mini-adaptation, it also sees the Electric Monk breaking into Douglas Adams's home to have a chat with Marvin The Paranoid Android. Meanwhile, Arthur Dent and Ford Prefect discover that they're both fictional characters, and, armed with this knowledge, they try to get Douglas to write another book. It was particularly sweet as Simon Jones, David Dixon, Stephen Moore and Peter Jones all reprised their roles from the 1981 Hitchhiker's Guide To The Galaxy TV series. The special ends with Arthur, Ford, Dirk and the Electric Monk all crowding around a grand piano breaking into an impassioned chorus of "I believe".

In 2005 a group of devoted fans got together via Douglas's own H2G2 website to produce and release an amateur audio play adapted from the novels, such was their love for the story and their desire for an adaptation.

But the only professional, complete adaptation of the stories was brought to BBC Radio 4 by Dirk Maggs, who also adapted the latter Hitchhiker novels for radio. Aired in 2007, it starred Harry Enfield as Dirk, Billy Boyd as Richard MacDuff and Olivia Colman as Janice, Dirk's frequently exasperated secretary.

As well as its all-star cast, the series boasts great performances. Billy Boyd is the perfect Richard (there's an air of Marty McFly about him), whilst Andrew Sachs was particularly well cast as Professor Chronotis. Jim Carter also made an excellent Detective Sergeant Gilks: a man who is plainly sick to the back teeth of Dirk's antics. Harry Enfield performs Dirk very much in the style of Michael Bywater's brief turn as the character - he's probably closest to Douglas's vision as it's possible to get.

Maggs (who both wrote and produced the series) kept very faithful to the novel, moving events around slightly but without changing the plot. Extra scenes added more drama for Michael Fenton Stevens as Michael Wenton-Weakes and Robert Duncan as Gordon Way, who were both brilliant in two very difficult performances to portray - especially on radio. The whole production is all the more impressive given the very descriptive and sprawling nature of Adams's prose.

Its success was such that an adaptation of The Long Dark Tea-Time Of The Soul aired the following year.

Whatever readers may make of the somewhat confusing paradox of the first novel's conclusion, it's clear there's something very special about the book. Written with great depth and understanding, it manages to be both hugely funny and at times deeply poignant. From the opening line ("This time there would be no witnesses") it feels like a classic novel - and that's because it is a classic novel. Douglas Adams will always be remembered for the sublime Hitchhiker's Guide To The Galaxy series but Dirk Gently's Holistic Detective Agency might just be his single greatest book.