Notably, this is a GUIDE, and firefighters carry one with them at all times, so you don't need to memorize it or study it like a textbook. Just be familiar with all of the sections, so you can look up information quickly.
The one exception is the "Firefighter Watch-Out Situations" and "Firefighter Standard Orders". These two lists should be memorized. The safety messages they contain were created by the National Wildfire Coordinating Group (NWCG) based on research into decades of fireline injuries and fatalities. If you can avoid these situations, you will be protected from MOST hazards.
However, sometimes you can't avoid these situations, that's why they're called "watch-outs," not "never do these."
As far as things you should know outside of the book:
One, I would say you should know that wildland firefighting is physically AND emotionally exhausting. The latter of which is often unacknowledged. Structural firefighting and EMS has taken significantly more interest in post-traumatic stress (PTS) recently, but wildland firefighting is years behind.
Two, be aware that this job can be taxing on your family as well. Wildland deployments are typically 14 days, not including travel, with a 48 hour break upon your return. In a busy fire season, this could translate to literally only a week or two at home during an entire summer. It's fun when you're young and single, and you make a lot of overtime pay, but it can be difficult (but not impossible, obviously some do it) to sustain over an entire career. I'd go into it planning to leave wildland firefighting at some point (if and when you're ready to start a family). After a few years of experience, you'll be well prepared for a career in structural firefighting, EMS, local "initial attack" wildland firefighting, forestry, forest ecology, landscaping, wildfire mitigation, park ranger, trail crews / public works, etc.
Again though, that's just my opinion, based on my experience. You may enjoy wildland firefighting so much, and have such a supportive and understanding family, that you could very well enjoy a 20-30 year career in wildland firefighting! It's up to you. Just make sure you take care of your, and your family's, mental health if you do. Good luck!