Lucinda's Secret (Spiderwick Chronicles, Book 3)

Things at the Spiderwick estate are getting completely out of hand. If being attacked by a band of marauding goblins and almost eaten by a bridge troll wasn't enough, how Jared is being harassed by Thimbletack, the house boggart, and Simon has a very hungry griffin recuperating in the carriage house. Mallory is convinced that the only way to stop the madness is to chuck the Guide. But that's not an option. With all kinds of creatures after them, the Guide is the only protection the Grace kids have. If they could just figure out why all of Faerie wants the Guide. Only one person can help solve this mystery--their crazy old Aunt Lucinda.