I loved One of our Thursdays is Missing, the sixth Thursday Next book; Fforde manages to keep the humor at a high level, something I think that very few humorous series authors manage to do. (I found Pratchett's discworld books to get a bit wearying after a while, and, of course, the Hitchhiker's Guide books are infamous for their humor falling off over time).
He's also not afraid to change the rules midstream, which may annoy other readers, but doesn't bother me in such a free-form series. In this novel, the whole story is built on the idea that the "actors" who play the characters are completely separate entities from the characters themselves, something that would make no sense in the earlier novels.
Overall, not a deep novel, but I haven't read an author so consistently funny whose name isn't P.G. Wodehouse.