Holly Black & Tony DiTerlizzi 
Seeing Stone [EPUB ebook] 

Thanks to the mysterious field guide left behind by their long-lost great-great-uncle Arthur Spiderwick, life for the Grace kids Jared, Simon, and Malloryis 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 its up to Mallory and Simon to save him, before its too late