Books don't really work without practical experience. I observed this in extreme abundance when I went through an automotive 'college.'
Basically, my timeline-
15-19: Worked on cars that belonged to me or my friends. Learned the most when I could be in one very specific area. E.g. carbureter needed work; learned all I could about Rochester Quadrajets and rebuilt/tuned mine. Distributor needed work; learned about GM distributor, from points to HEI, learn to set base timing, mechanical and vac advances. Ect.
19-20: Start working at a shop. Realized I didn't know dick. Slowly learned what I needed to work my way up to a low level tech. Did brakes, alignments, ect for the most part
20-21: Went to school. Realized I didn't know dick. But I was lightyears ahead of the kids coming out of high school, and I was able to properly absorb all the information that was taught, and actually apply it to reality.
21+: Started working as a higher level tech. Realized I didn't know dick. Slowly gained comfort and familiarity with most all aspects of a typical car. Eventually got to the point where I could trust my own knowledge/judgement/ingenuity and pretty much fix whatever I wanted to, with increasingly limited incidence of 'oops I dun goofed' moments.
I still make mistakes.
If you think you can just say 'I'm going to learn to fix cars' and pick things up quickly by reading a textbook, you're headed for disappointment and/or death-by-car.