Find a mentor you trust. Not the loudest guy, not the flashiest guy but a solid official who won't BS you. I almost hated the guy that trained me as a wingman but he made me a better official.
Buy the Reddings Fed guide. It breaks the rulebook up into a much easier to read and understand format. The rulebook can read like the tax code and I've always hated the casebook, particularly last year when there were so many revisions. The casebook reads like the old math word problems I hated in school - "a train leaves Chicago with a load of apples at 4:00 pm, another train leaves LA with a load of bananas at 6:30. How many bananas did the monkey eat?"
If you can find video to watch, get it. The one thing that I've been disappointed in is the product that the NF puts out. They have a annual training VHS tape, I think its about $40 and it doesn't tell you much. See if your local coaches will let you look at some tapes. Learn to watch it as an official, not a fan.