The Wrath of Mulgarath - Holly Black, Tony DiTerlizzi

Twists and turns abound in the final installment of the Spiderwick Chronicles. Bruised and battered, Jared, Simon and Mallory return home to find their house completely ransacked and discover that Mulgarath has made off with their mother and Spiderwick's Guide. It's up to the Grace kids, with only the help of Thimbletack, Hogsqueal and Byron, to defeat the evil Mulgarath and his goblin army. But first they have to rescue Arthur Spiderwick from Lorengorm and the elves!Their world is closer than you think... their world has arrived!