The Field Guide (Spiderwick Chronicles #01 )

After their parents' divorce, the Grace kids - twins Jared and Simon and their older sister Mallory - move with their mom to the decaying old mansion belonging to their ancient great-aunt Lucinda, who has been sent off to an asylum.

But before they can even unpack, Jared begins to notice that they're not alone in the house.

There's something living there with them. His investigations eventually lead to an old book filled with pictures of fantastic creatures "Arthur Spiderwick's Field Guide to the Fantastical World Around You" - and an even bigger mystery - the disappearance of his great-great-uncle and Lucinda's father, Arthur Spiderwick.