If you’re interested to learn, just buy the book (or maybe subscribe to the website as others have suggested). $25 doesn’t strike me as very expensive for learning something new.
Regarding bike fit, especially on a MTB, I think you should just ride it and make adjustments if you feel something is off. For MTB your style of riding influences your setup a lot, and I can’t see how anyone can tell what’s right without seeing you ride. But I’m sure I’m in the minority with that view.