The Wrath of Mulgarath (The Spiderwick Chronicles, Book 5)

After narrowly escaping the dwarven quarry, a battered and bruised Jared, Simon, and Mallory return home to find it has been ransacked by the evil Mulgarath, and that he's made off with Arthur Spiderwick's Field Guide to the Fantastical World Around You along with something far more precious to the Grace kids -- their mother. With the help of Thimbletack, Hogsqueal, and Byron, the Grace kids have to figure out how to rescue Arthur Spiderwick from the domain of elves so that he can help them stop Mulgarath and defeat his goblin army before it's too late.