The Spiderwick Chronicles, Volume III, Book 5, The Wrath of Mulgarath

Book 5
Bruised and battered, Jared, Simon, and Mallory return home to find their home ransacked and learn that Mulgarath has made off with their mother and Spiderwick's Guide. With only the help of Thimbletack, Hogsqueal and Byron, the Grace kids have to defeat Mulgarath and his goblin army. But they have to rescue Arthur Spiderwick from Lorengorn and the elves first.