I just finished my first year and I honestly didn't find the Simplified and Illustrated book that useful. A study guide I would highly recommend is the Redding Study Guide for NFHS Rules.
You can purchase it at
www.honigs.com or
www.doublesdistributors.com .
Also, check out
www.usafootball.com . They have some good free material on there and I hear the material on the paid side is very good as well.
Otherwise, I just spent some time reading the forums and going over plays with fellow officials. I obviously have much to learn but I feel I had a pretty good start.