The book is a kind of surrealist satire. It's symbolically rich and concerned with truth and identity and revolves around the discovery of a conspiracy, although it is not absolutely clear that the conspiracy actually exists.
The book is very readable. Pynchon's prose is deeply enjoyable and he manipulates words for their symbolic value expertly.
I am a little tired of this sort of thing though. The book is as well written as this sort of thing can be. He reminds me of a better Martin Amis, although I am sure the influence runs the other way.
Good if you like this sort of thing, but I am not deeply enamored of it.
Update: This is a book which gets better the more you think about it. I'm upping its rating one star.