The Field Guide: The Spiderwick Chronicles, Book 1 by Tony DiTerlizzi and Holly Black. Website at The Spiderwick Chronicles.
The Plot: Mallory (13), Simon (9) and Jared (9) Grace, and their mother have moved into the run down Spiderwick Estate. As they unpack and explore the house, the kids find more than they bargained for: faeries. And these faeries aren't cute and helpful creatures.
The Good: this is a cute, charming story; yes, cute, even tho the faeries cause trouble. It's in the tradition of The Borrowers and The Littles, with hints of secrets and darkness.
I loved the world-building, with maps and drawings and tons of details. I'm sure this has inspired tons of fanfiction in the upper elementary school crowd. It's also a beautifully packaged book, with short chapters. Because of the world building and story, it's sophisticated; yet the language, length of the book, and short chapters also makes this an easy read for younger kids.
The three siblings are realistic; they fight, but they also band together when necessary. And it has my favorite: kids thinking, working together, figuring out things for themselves, being active.