The first four books (The Eyre Affair through Something Rotten) pretty much all follow one after the other; the first one is a brilliant, wacky, comic excursion into an alternate reality, and the next three are kind of a set. I mean, the first one ties up the plot threads quite neatly, but then it apparently did so well that Fforde just went all-out and set up LOADS of plots in the second book, which required another three books to deal with. Bit different from a standard trilogy set-up, frankly.
So, at the end of the fourth book, intrepid Literary Detective (in the "real world") and Jurisfiction Agent (in the "Bookworld") Thursday Next has finally gotten her husband back, and she and their son Friday are all settled in, etc., etc. and the crazy grammatical plots in the Bookworld and the crazy political plots in the Real World have been dealt with. Yay.
Book Five... jumps forward about sixteen years, hints at plot threads from earlier books and how they got tied up in the meantime (which, I guess, leaves space for other books), and deals with an entirely new set of problems, mostly having to do with Thursday and Landon's teenaged son.
Now, the thing is, in the Fforde-verse, books have a very tangible reality, their own world, their own rules, their own police force (which Thursday works for, occasionally). Characters from books? They are real. Thursday started out working partnered with Miss Havisham; even before that, she was interacting with the cast of Jane Eyre; Jurisfiction as a whole uses a deserted ballroom at Norland; there's a problem with an escaped Minotaur hiding out in Zane Grey novels.
In order to raise money, in the sixteen years since the previous book, Thursday finally caved to pressure and sold the rights to her adventures. Which means that they have been published. Which... means that Thursday has written versions of herself to deal with.
And then, in the sixth book, the main character is one of the written versions, who has to step in when the real Thursday Next turns up missing.
[O.o] - O rly?
I am not sure exactly how I feel about that-- it does interesting things with the series, and the universe, and the general concept of the Bookworld, and it does get away from the fact that the fifth book brought the "real" Thursday's story to pretty much as conclusive a conclusion as it looks like it's going to get. On the other hand.... yeah, through meta, out the other side, giant martini.