Lucinda's Secret (The Spiderwick Chronicles, #3) by Tony DiTerlizzi - old paperback
As if being attacked by goblins and a bridge troll weren't enough, Jared is now being targeted by Thimbletack. Simon is keeping an injured and very hungry griffin hidden in the carriage house. And Mallory is convinced the only way to get things back to normal is to get rid of the Guide. But that doesn't seem to be an option. With more creatures from Arthur Spiderwick's Field Guide to the Fantastical World Around You popping up, the Guide seems to be the only protection the Grace kids have. But why do the faeries want it so badly? There's only one person to go to for answers -- crazy, old Aunt Lucinda!