One of the best overall books which I've found is Walther Howe's Professional Gunsmithing. Yes it's old, but it covers a lot of the basic concepts.
One thing to remember, gunsmithing isn't what it use to be. In the old days, the gunsmith worked on everything: wood, metal, rifle, shotgun and pistol. Today, they're more specialised: pistolsmith, riflesmith, stockmaker, etc. If you want to go that route, then pick the topic and start working on it.
I also suggest taking those NRA summer classes. I've taken quite a number of them and they provide a good basis to understand operating principles, the mechanics of disassembly and reassembly of firearms. Afterwards, I've taken numerous factory armorers' schools and while they're good (and you'll learn factory standards), they never approach the level of the NRA program.
Go slow and finding a mentor helps.
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