Grade school? I really liked the Animorphs by K.A. Applegate. Great characters, great concept, and a great storyline. Not such a great ending though, but still a worthwhile read. Kept me busy too. I think there are 60 books in the series.
Eh, I was addicted to Kathryn Lasky's "Guardians Of Ga'Hoole" series. It had the edginess that kept me hooked and they never were boring. I don't really read that much now though.