The Spiderwick Chronicles, Books 1 and 2

The Field Guide:
The Grace Kids—twins Jared and Simon and their older sister, Mallory—aren't all that impressed with the ramshackle old mansion their mother moves them to when their decidedly kooky Great Aunt Lucinda is sent to an asylum. But a series of mysterious events within the house leads the three young Graces to an old book filled with hand drawn pictures of all kinds of fantastic creatures. Jared believes the book holds the key to all the freaky goings on (for which he has been blamed).
The Seeing Stone:
The Grace kids are just beginning to get used to Aunt Lucinda's strange old mansion when Simon suddenly disappears. At first, Mallory thinks that her younger brother Simon is pulling some kind of prank. But when it becomes obvious that he's been snatched by goblins, Jared and his sister have to rely on the help of a mischievous house boggart, a nasty bridge troll, and a loud mouthed hobgoblin to get him back—and uncover why he was snatched in the first place!