Since the prevailing opinion is that we are headed back at least three generations in time and it is a good idea to seek out technology fromt hat era, may I suggest that blacksmithing is an important subject of research. I currently have about a dozen books on this subject and a moderate forge besides. Even so, it takes a long time to master even the basic skills needed to transform scrap into useful tooling. The reason that this field is so important is precisely because the blacksmith was THE place to get an item made or repaired if it was made of metal. Furthermore, it was the smith that produced the tooling needed to sustain the small scale agriculture and industry that existed at that time.
I could list many books, but I will offer one individual as a go-to source. His books are still available in used markets and they are worth every penny spent to obtain them.
https://en.wikipedia.org/wiki/Alexander_WeygersAlexander Weygers was a European trained machinist and an artist that turned to the anvil to supply the tools he needed for sculpture and other avocations. His drawings are clear and exact and they litter each page of clear and exact how-to. From scrap yard to final tempering, Weygers pretty much covers it. Any backyard smith with a homemade forge and a piece of railroad rail for an anvil will be able to pretty much produce whatever tool is needed within basic trade limits.
Become a toolmaker and you will rule the post-apocalyptic world.