The Great Good Thing, by Roderick Townley

Sylvie lives in a fairy tale world -- a real fairy tale -- in fact, she lives within the pages of a book. When the book is closed, she's free to explore the pages and go where she likes. But when the book opens and the Reader appears, she has a role to play and a story to tell. She may be a princess, but she's only a character in a book. All of that changes when she violates the rules and meets the Reader. And when disaster strikes her world, Sylvie leads her people on an epic voyage into the world beyond.

This is a clever, although a bit surrealistic fantasy book. Geared towards middle readers, it has some pretty tricky plot twists that may confuse some younger readers and turn them off. But for those who enjoy thinking-person's fantasy, this is a fun little read.