The Ironwood Tree by Tony DiTerlizzi and Holly Black

The Ironwood Tree
Tony DiTerlizzi and Holly Black
Publish year:
Book four of the Spiderwick Chronicles. After a trip to Mallory's fencing tournament ends up with Jared in serious trouble, it is discovered that Mallory has disappeared. In their search for their older sister, Jared and Simon find themselves being taken hostage at the hands of a group of dwarves. Unbeknownst to the twins, these dwarves have been employed by power-hungry ogre Mulgarath to get possession of the coveted field guide that the twins once had possession of, as well as encasing Mallory in a glass coffin that will keep her immortal as long as she doesn't leave the case and to prevent Mulgarath from executing his plan to take over the world, they must escape with the guide and their sister.