That is actually a task these days. Most everything in print the last 20 years is an ad for some crap the aftermarket wants to sell. We have a massive used book store here, and they are hard to find there. The best place is swap meets, but fleabay and amazon have some. Here are some in my library, I have a ton of out of print books, I grab them when ever I see one.
Peterson publications had great paperbacks with hot rod articles reprinted, there is excellent tech in there. Mopar put out 2 books to encompass all types of modification of their cars, they are some of the best primers out there, out of print of course. The engineers flat out told you, if you want a duster to go 11 flat, here is how to do it. The best book I've seen on engine performance theory is smokey yunick's power secrets, out of print. One thing he touches on is dwell time/rod ratio. The longer the piston stays at the tdc, the more force can be generated as the fuel burns. Basically a longer rod vs stroke, mopars had the best ratio. He argued with the gm engineers and they blew him off. Mopar also had shaft mounted rockers that "float" on an oil bearing, vs bolt through rockers that are a fulcrum. It's little shit like that that made mopar so strong for so many years. Virtually all race engines now use needle bearing roller rockers.
And lastly, factory automotive service manuals. Not exactly a fun read, but they do break down how all the components work and interact. Reading this crap will give you an excellent head start to diagnostics, it's much easier when you know the flow and what systems are in plan. I like the books, ford makes them generally in 5 books, you need all of them. Ebay will have manuals on dvd, they are likely stolen from service departments and copied. I really should have done that while I was there, didn't think I'd need them.
You don't mention where you are from, that makes big differences in the answers. In general, people start messing with things like lawnmower, motorcycles to get into mechanics. I went to school for body, then mechanics. The mechanic coarse was broken down to 4 parts, or semesters. Transmissions, tune up, engines & ac, suspensions & brakes. Now days things are way more complicated, but those are the 4 core things to learn. Engines and tune up are the most intense requiring a lot of understanding, but essential. The rest is mostly parts changing, transmissions are not that difficult, the new ones are a pain in the ass because of all the electronics involved.
So where to turn to find this stuff? Service manuals are an excellent way to learn how cars work. I like the ford stuff. General manuals are not very good at all for learning, chilton, mitchell, clymer. Back in the day, magazines used to have great tech articles and were a wealth of info before internet. Now most magazines are just shills for ads. Peterson had a great catalog of reprints from their magazines, all out of print but you can find them. Here are some of the books I have, basic electric and how to tune are very good. You really should be good with electricity especially on British cars. There is nothing unique to these old cars, they are super simple to deal with. As far as how engines work, the best book I have ever seen is by smokey yunick, I don't know why they are so expensive now. Mopar put out a series of books to help people race their cars, also an excellent source if info. You can find this stuff at used book stores, swap meets, I grab everything I can find. The theory helps overcome the trepidation, once you have a basic knowledge of what everything does, it makes the actual physical work easier.
Body, this is where art comes in. It takes years of practice to become a bodyman. It truly is a highly skilled craft. It is where the most time and money is involved when working on cars. This is really where you start a project, if the body is too far gone, there is no point in going further.
Apart from getting a little project, what can you do? And remember this is a very expensive endeavor. You can find car clubs in most communities, you generally won't find many that even have a clue, but it gets you in the door and you might offer to help. A lot of them just sit around and drink beer and never do any actual work on cars. The misinformation in these car groups is staggering, this is where books help. Euro cars are a niche where the owners are a bit on the crazy side, they at least work on the cars. In /r/littlebritishcars people might direct you to such groups. The volvo people as well. Remember, it's all the same, just different name plate. Reddit's search sucks, but there are lots of posts in this sub about exactly what you are talking about. MGA's are a blast to drive.