Workshop manuals won't tell you how do modify anything...
But you need to know what something is and how to take it apart before you modify it...
A TAFE course would be a good choice, they have courses for people who know very little and want to learn. Another good (well essential really) option is to simply buy a car and start playing with it. I'd suggest an E-Series because they're cheap as, information and parts are everywhere for them, they have very little rust problems and they're easy to work on, apart from the low bonnet height...
IMO the hardest part about learning about something (anything, not just cars) is sorting out the bullshit from the facts. The problem there is that everyone owns a car, so everyone assumes they are a mechanical engineer, so you get so much more bullshit than most other things.
A lot of people who work on cars for a job are only taught by one or 2 people as well and think because they're professionals (they get paid for it) they know everything. Not saying all mechanics are dumb, but quite a few are.
Anyone who tells you that old cars are heaps easier to work on than new cars is a redneck who is afraid of technology. Newer cars (EA-onwards) were designed to be assembled as quickly and cheaply as possible on the production line, so pretty much everything bolts/clips in. Once you work that part out, and accept the fact its generally easier to replace a part rather than repair it, working on newer cars gets a whole lot easier.