Fablehaven by Brandon Mull

Kendra and her brother, Seth, have no idea that their grandfather is the current caretaker of Fablehaven. Inside the gated woods, ancient laws keep relative order among the greedy trolls, mischievous satyrs, plotting witches, spiteful imps, and jealous fairies. But when the rules get broken, powerful forces are unleashed, and Kendra and her brother face the greatest challenge of their lives. To save their family, Fablehaven, and maybe even the world, Kendra and Seth must find the courage to do what they fear most...