Seven-Day Magic — Edward Eager

All books are magic, but some are more magical than others. When Susan opens a strange library book, she discovers it is about her and her friends, leading up to the moment when she opened the book. Beyond that, the pages are blank... waiting for the children to wish the book full of adventures.

Fredericka asks for wizards and beasties, and a dragon carries her off. Susan journeys to the world of Half Magic, and finds that mixing magic creates trouble -- far too much to deal with before the book is due back at the library.

Wishing, it seems, is a tricky business. And if the children make the wrong wish, their adventure won't end, as all stories must, happily ever after.