The Spiderwick Chronicles: Field Guide by Tony DiTerlizzi (Book 1)

After finding a mysterious, handmade field guide in the attic of the ramshackle old mansion they’ve just moved into, Jared; his twin brother, Simon; and their older sister, Mallory, discover that there’s a magical and maybe dangerous world existing parallel to their own—the world of faerie. The Grace children want to share their story, but the faeries will do everything possible to stop them...