Douglas Adams is one of the best writers of humorous Science Fiction (which is how I am classifying this, though you could, I suppose, call it Fantasy). The interesting thing is that Dirk Gently does not appear until about the mid-point of the book, though he is mentioned in passing a few times earlier. And what Adams does here is to take a number of bizarre threads and tie them all together into a story that makes a weird kind of sense when it is all done. And Dirk ends up being smarter than you first think. I plan to read the follow-up, Long Dark Tea-Time of the Soul.