The Spiderwick Chronicles

the long version

Thanks to the mysterious field guide left behind by their long-lost great-great-unckle Arthur Spiderwick, life for the Grace kids--Jared, Simon and Mallory--is beyond weird. When Simon goes missing, Jared is convinced creatures from the faerie world have something to do with it. Mallory is not convinced. That is, until she and Jared have to contend with a band of menacing, marauding goblins. Simon is clearly in danger and it's up to Mallory and Jared to save him. (from the cover)