The Wizard's War (Key Hunters #4) : Volume 4 (Key Hunters)

This time Cleo, Evan, and Ms. Crowley (who, it turns out, is the missing Ms. Hilliard's sister) find themselves in a confusing fantasy book, caught up in a war with wizards, elves, trolls, and the mighty Golden Dragon--and while they manage to find theirmissing librarian, their quest does not end there.