I picked up a complete encyclopedia set from 1937 at a gun show after I leafed though a few volumes and realized it was like holding a complete set of instruction manuals for jump-starting civilization from scratch. Everything. Detailed drawings and formulas for everything from steam power generation to chemistry (black powder, nitrocellulose, and azides for primers, etc.) and electric power generation. It's also why I started eyeballing the steam tug and locomotive on display at the railroad museum with an eye towards what it would take to put them back into service. The tug for electric power generation and put the locomotive back on the local rail line for moving goods and services.