Welcome to the dark side.
As you've already seen, the NFHS rulebook isn't the easiest thing to read. Break it into chunks. With the exception of Rule 2, most of the rules are less than 10 pages long. Study them one at a time. "Today, I'm going to spend all of my study time on Rule 6." As you read the rules, ask yourself how you'd explain that rule to a player, or coach, or casual football fan. If you can explain it in your own words, you've got it.
Study the Case Book as well. It's easier to read than the rule book and makes many of the rules more clear with real game examples or situations.
Google "Aloha Football Clinic" and watch the videos that they've posted. This will help you with the letter of the rule versus the practical application (no, they are not the same thing).
Find a mentor. Shadow a crew that is willing to let you ask questions, sit in on their pregame and halftime sessions, and go to the post-game debrief (aka beer & burgers).
Find a friend. Get to know another new official, and study together. Explain rules and interpretations to each other. Give each other quiz questions.
Try to ask questions that show you've been studying. Don't ask "Is it legal to trip the runner?" Ask "The defender can trip the runner, right?" One question shows you've been studying, the other doesn't.
Verify what you're told. If you get conflicting information from different veteran officials (and you certainly will), don't believe either/any of them. Look it up. That'll teach you the rule, and will also let you know who you should stop asking questions to.
Reddings - already been said. It's about $12 and you can get away with only buying it every other year.
Hang out on this site, and pay very close attention to what Ralph says. Unless he's talking about his beloved Sox or Pats.