The Seeing Stone

The second book in the Spiderwick Chronicles.
 
Thanks to a mysterious book left behind by Arthur Spiderwick, life for the Grace children is stranger than ever. And, when Simon goes missing, Jared is convinced his brother has been kidnapped by creatures from the faerie world.

Their sister, Mallory, is doubtful... until the siblings are faced with a band of menacing goblins. Can Jared and Mallory save Simon before it's too late?