AF, OTW, and SIM are prefixes applied to the callbacks (real 'nuts and bolts' things). I think they stand for "air frame" "of the world" and "simulator." Views will be OTW, landing gear would be AF, and an OSB would sim if you get the pattern. It's not too important to understand this and just X callback does Y action is more than enough knowledge. The callbacks should have plain English descriptions associated with them.

As for a cover-to-cover textbook "learn BMS" where you pick it up knowing nothing and put it down knowing everything, it doesn't exist. It's been discussed quite a bit but no one has volunteered to construct such a thing. As for knowing all the systems, my suggestion is to start on the left side the cockpit with a magnifying glass and study each one in depth until you get to the right side. Eat the elephant in spoonfuls. Fly the training TEs in order and read the original F4 manual descriptions for the ones that have them. Ask lots of questions. Do searches. You'll know a lot more after doing all the training TEs to a high level of proficiency.