The Wizard's War — Eric Luper

Cleo and Evan have a secret. A collection of books so dangerous they are locked up tight. A friend has vanished inside the pages of one of them. It's up to them to find the key that will set her free.

A MAGICAL KINGDOM AT WAR
The quest is clear. To save the kingdom, Cleo and Evan must battle clever elves, horrible trolls, and the mighty Golden Dragon. Magic will help them win the war--and find the right key. But it will take more than swords and spells to survive this epic fantasy