I have multiples here - going to suggest you consider what you mean by "gunsmithing". For example - 1920's books by Clyde Baker or James V. Howe - chapters on gunstock checkering start with how to make your checkering tools - that sort of approach. How to check and straighten a rifled barrel. Lots of stuff on old school metal work - heat treat, draw temper, etc. Similar to 1950's Roy Dunlap - for a lot of his books, he assumed you knew how to deal with steel - make your own parts with file, etc. Assumed that you could weld, use a heat sink, run a milling machine, etc. Is a lot of projects about "customizing" the cheap milsurps that would have been available then. I did not find "modern" books like by Bryce Towsely or Patrick Sweeney to be very useful - might be interesting to someone who does not know how to file or to grind a screw driver for slotted screws. Jim Carmicheal had some good ones - from the same era as Dunlap, I think.
Is about difference of a "gun plumber" - screwing on parts from a package like at the counter at a gun store, versus a military armourer who has specified procedures to follow and a vast supply chain feeding his work, on a very few firearms, versus a gunsmith who can make a replacement for a failed part, including springs, etc. In the end, I think gunsmith trade is about making a profit to keep the doors open - so like bookkeeper and accountant - a gunsmith likely makes most of his money doing "gun plumber" type jobs - no body today wants to really pay for what a "gunsmith" can actually, or should be able to, do.