When Mallory and Jared attempt to rescue Simon from goblins, they use a magical stone which enables them to see things that are normally invisible.

With goblins, trolls, and the house boggart all trying to get them, the Grace children turn to Great Aunt Lucinda for help.

Book 3—Lucinda's Secret
Things at the Spiderwick estate are getting completely out of hand. If being attacked by a band of marauding goblins and almost eaten by a bridge troll wasn't enough, how Jared is being harassed by Thimbletack, the house boggart, and Simon has a very hungry griffin recuperating in the carriage house. Mallory is convinced that the only way to stop the madness is to chuck the Guide. But that's not an option. With all...

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.

First a pack of vile, smelly goblins snatches Simon. Then a band of elves tries to entrap Jared. Maybe now that Arthur Spiderwick's Field Guide to the Fantastical World Around You has disappeared, the Grace kids will be left alone. Don't bet on it. At school, someone is running around pretending to be Jared, and it's not Simon. To make matters even worse, Mallory has disappeared. Clues point to the abandoned quarry just outside of town. The dwarves...