Jasper Fforde's Thursday Next series is one of my all-time favorite collection of books. It's such a mix of literary references, fun, mystery, and fantasy - there's something new at every turn.
The sixth book in the series finds us again in the BookWorld which has finally been remade to be geographically similar to a planet rather than a library - so helpful. The real Thursday Next, who divides her time between working in the real world and BookWorld, is missing and the written Thursday Next, through a few of classic plot devices, gets on the case to try and find her. Of course, the BookWorld is so literal that nothing is simple to wrap up, but the real Thursday has to be found or else war might break out between the islands of Racy Novel and Women's Fiction (which in itself is a funny thought - porn characters vs fictional women's lib advocates.)
The great thing about this novel is the subtlety. Literary references, plot devices, writing techniques are all major components in the BookWorld that Fforde subtly and humorously weaves into the mystery story of locating the real Thursday and discovering why someone wanted her dead in the first place. Simple things like naming a cab driver Mediocre Gatsby (the brother of Great Gatsby) and having roads and squares named after authors who have found great success are just a few examples of how Fforde weaves the entire lexicon of literature into his stories.
An even deeper layer to this story is the character development of the written Thursday Next. She's battling her insecurities and uncertainties, pushing back against previous failures to really prove herself as an individual (while also staying true to the written version of a real person.)
The series is engaging and exciting and so much fun to read. Definitely start at the beginning though if you're new to Fforde with The Eyre Affair and enjoy!