The main thrust of the book is not Dirk Gently, or his holistic detective agency, but a young man named Richard MacDuff who has found himself embroiled in a plot featuring time-travelers, ghosts from a bunch of different eras, and various things which have been misplaced in space. Eventually, when the plot really gets moving, Richard does meet up with Dirk Gently, and things become both increasingly more bizarre and they start making more sense.
The book is quite good, which is why I'm not going to go into the plot at any great length, and if you're a fan of Douglas Adams' other works, then you'll probably enjoy this initial (and sadly penultimate) adventure of Dirk Gently. It's clever, with a good bit of mystique, and it shares several plot points with old Doctor Who serials, which is actually quite unsurprising.
I may not continue doing these book reviews, as I seem to be downright terrible at them.