Two books in one (Dirk Gently's Hollistic Detective Agency and The Long Dark Tea-Time Of The Soul), this is a not-so-sci-fi-but-at-least-as-strange offering from the much-loved and much-missed Douglas Adams (of Hitchhiker's Guide To The Galaxy fame, for those of you in the dark). Even more so than in the "Hitchhiker's..." books, the focus of both of these is on more than one main character. There's the wholly eccentric Dirk Gently, who only discovered his skills by accident when he was at university (and subsequently had to pull a disappearing act), an electric monk, a very old university professor, Thor (he of mythology), a greedy lawyer and his wife, and an eagle with nasty claws, amongst others.
In the first book, Dirk gently's Hollistic Detective Agency, Dirk pops up when an old friend of his starts acting strangely. There's also the little matter of the strange sofa (it was small enough to get into the stairwell, but then too big to get out), an unsolved murder, a ghost, an electric monk that's on the loose, and not forgetting what a certain professor's been up to in his spare time (travelling). Adams manages to weave a number of suitable strange and initially apparently unconnected events into a pretty good story. The humour, well, I personally enjoy it and find it funny, but i can see how it is more of an acquired taste than that of the "Hitchhiker's..." books - it relies more on weirdness (if that's possible).
Some people may get frustrated that most of the threads don't really come together until about two thirds/three quarters of the way through the story (a pattern followed to a lesser degree by the second book in this omnibus) - and i seriously doubt you'll be able to solve the mystery unless you already have an idea of what it is before you start reading.