Spiderwick Chronicles: Wrath of Mulgarath Book 5 by Tony DiTerlizzi

Bruised and battered, Jared, Simon, and Mallory return home to find the mansion ransacked and learn that Mulgarath has made off with both the Guide and their mother! With only the help of Thimbletack, Hogsqueal, and Byron, the Grace kids must defeat Mulgarath and his goblin army. But before they can face Mulgarath, they’ll have to rescue Arthur Spiderwick from Lorengorm and the elves.