I read about two thirds of this book, until the little group figured out everything that was going on in about five pages of exposition.
I really miss when Mercedes Lackey wrote for adults. I think this book wouldn't be too far out of the range of something an eight year old friend of mine who's steadily working her way through the Boxcar Children could read, aside from the subject matter of zombies.
VERY young adult. Your mileage may vary.