This is the fourth book in the Fforde's series about Thursday Next, a literary detective. The books are set in an alternative universe, where history is somewhat different, and the boundaries between fiction and reality merge. Fforde's world is surreal but once you delve in it takes on its own life, leaving you chuckling away. There's word play, clever manipulation and reorganisation of reality, and allusions to all sorts of classics and literary concerns. It's an intelligent read, but also requires some flexibility in the mind of the reader to accept all sorts of ridiculous ideas (which have a strange logic of their own). Once you do so, then you're hooked!
The plot takes you through the fiction world and the "real world" as Thursday tries to maintain law and order.
Many of the premises, characters and in-jokes have developed from earlier books, so any reader is best advised to start with the first in the series: The Eyre Affair.