The Spiderwick chronicles

It all started with a mysterious letter left at a tiny bookstore for authors Tony DiTerlizzi and Holly Black. It's closing lines: "We just wanted people to know about this. The stuff that has happened to us could happen to anyone." Little could they imagine the remarkable adventure that awaited them as they followed Jared, Simon, and Mallory Grace and a strange old book into a world filled with elves, goblins, dwarves, trolls, and a fantastical menagerie of other creatures. The oddest part is in entering that world, they didn't leave this one!