The Field Guide (Book 1 of The Spiderwick Chronicles), by Tony DiTerlizzi and Holly Black, 2003

What happens when a brownie goes bad (and it's not a chocolate cookie bar)? It becomes a boggart, a tiny mischievous creature who creeps between walls and under beds, stealing and spreading mayhem and getting others blamed for it. Jared Grace knows this, and eventually, so does his twin brother Simon and even their older sister Mallory. Clues suggest that their Spiderwick ancestors knew even more. But Mom refuses to believe anyone but Jared could have ransacked the kitchen and hogtied Mallory's hair. After all, Jared has been acting up a lot since the divorce ... This first of the Chronicles reads more like a serial installment than a complete novel, but its size, complexity, and illustrations make it well-suited to 7-9 year old readers.