Well, we knew that you couldn't base an entire TV series on just two books (and the first chapters of a third one) but even the 60 minutes long TV pilot won't be a straight adaptation of the books as SFX releals in an exclusive interview with writer Howard Overman.
“I’m aware that it’s a bit of a poison chalice... What I did was take the character, Dirk Gently, and his detection method, and write a new story. It has elements from the book, obviously, in it, but it’s kind of a different story. Because if you’ve ever read the book, the story in there isn’t really adaptable for TV. Especially not on the budget we’ve been made to do it on, because it involves an alien planet and all sorts of weird and wacky ideas."
So will it be an understated adaptation with just a bit of Douglas Adams (Dirk Gently and his quirky and weird methods) and a more traditional (and less costly) detective story ? Well we certainly hope that this Dirk Gently adatpation won't be too classic even if doesn't involve an alien planet!