Rise of the Evening Star by Brandon Mull

At the end of the school year, Kendra and her brother Seth find themselves racing back to Fablehaven, a refuge for mythical and magical creatures. Grandpa Sorenson, the caretaker, invites three specialists — a potion master, a magical relics collector, and a mystical creature trapper — to help protect the property from the Society of the Evening Star, an ancient organization determined to infiltrate the preserve and steal a hidden artifact of great power. Time is running out. The Evening Star is storming the gates. If the artifact falls into the wrong hands, it could mean the downfall of other preserves and possibly the world. Will Kendra learn to use her fairy gifts in time? Will Seth stay out of trouble?