The Ironwood Tree (The Spiderwick Chronicles, #4)

First a pack of vile, smelly goblins snatch Simon. Then a band of elves try to entrap Jared. Why is the entire faerie world so eager to get their hands on Spiderwick's Guide? And will the Grace kids be left alone, now that the Guide has mysteriously disappeared? Don't count on it.
At school, someone is running around pretending to be Jared, and it's not Simon. To make matters even worse, now Mallory has disappeared and something foul in the water is killing off all the plants and animals for miles around. Clues point to the old abandoned quarry, just outside of town. Dwarves have taken over an abandoned mine there. And the faerie world's abuzz with the news that a creature with plans to rule the world has offered them a gift to join with him -- he's given them a queen...