The Forgotten Book by Mechthild Glaser is about a school set in an old castle where the students find a world of adventure and intrigue. When they start exploring the passageways, they locate an old library in an unused wing of the castle. As they work to turn this into a special gathering place, they find an unusual, old book. The book has not yet been written, and they don't realize at first, but as they write in the book, their words become reality. With the reality of their words, also comes the consequences. So, when faced with the possible death of a fellow student, they have to rewrite the future. This book has lots of adventure and excitement and a lesson about the power of words.
If you enjoy The Forgotten Book, you might also try The Book Jumper written by Mechthild Glaser.