The Seeing Stone (Spiderwick Chronicles, Bk. 2)

Thanks to the mysterious book left behind by their long-lost great-great-uncle Arthur Spiderwick, life for the Grace kids - Jared, Simon, and Mallory - becomes more unpredictable than ever. When Simon goes off in search of his missing kitten and vanishes, Jared is convinced that one of the creatures from Arthur Spiderwick's Field Guide to the Fantastical World Around You has something to do with it. Mallory is not convinced. But when she and Jared have to contend with a band of marauding goblins, she begins to think Jared might be right - Simon is in grave danger and it's up to her and Jared to save him.