Very good point.
I am always looking for old books from the Great Depression era (the 1929 one, not Obama's) and the WWII era that explain how to do things from scratch using basic tools, supplies and ingredients.
The old recipe books are useful because they were written in a time before the availability of highly processed foods and ingredients were taken for granted. They often contain substitution tables and hints on how to work around shortages.
I often find books like these in the free bins or at low prices - almost no one wants them because of their age and "outdated" content but that is exactly why they are valuable to a prepper.
In my experience two good sources for older volumes at low prices have been low end flea market booksellers and the "Friends of the Library" book sales.
One of my treasures is a 1930s/1940s industrial chemistry book.