There's lot out there, many are old timers so look on Amazon used book sellers... Brownells 'Gunsmith Kinks I, II, III, IV" have lots of tips, there are many specialized books for 1911, AR's, S&W, military surplus renovations and pistol smithing in general, stocks, etc.
A lot of the knowledge deals with machinist skills such as mills, lathes, metal working and hardening. A couple machinist handbooks could be invaluable. Learn to blue... hot, cold and rust... and other finishes. Learn to work & finish wood... so much to learn about...