Youth Fiction by Brandon Mull.
I've been reading this book to the twins, who are about to turn nine.
It was unexpectedly scary at the beginning, since I picked it up at the thrift store and didn't know what it was about. It started out as a regular sixth-grade kid going to school with his regular sixth-grade friends, and then WHOA! BAD THINGS started happening. The twins were shocked. (So was I.)
And for those of you who think I should have known what was coming from the scary-looking cover --what IS that crazy black crab thing?!-- remember, I got it from the thrift store. It had a much calmer cover, I promise.
Anyways, it's pretty good so far. I might keep reading the series even if the twins lose interest.