Fablehaven: Rise of the Evening Star by Brandon Mull

In Fablehaven: Rise of the Evening Star (book 2) by Brandon Mull Seth and Kendra Sorenson are off on another adventure when a strange kid shows up at their school.

Kendra can see that he's not human, but other people can't see past the disguise he's created. Who is he, and why is he there? This sets off a series of events that puts Seth in the path of a gluttonous monster, and the siblings off on another trip to Fablehaven, their grandparents' magical creature preserve, populated by creatures both dangerous and benign, but probably the safest place for them right now. Will the boundaries of the preserve keep the gluttonous monster away from Seth? But what if they're a traitor in their company? And what will happen to Seth if the monster gets in?