The Joy of Cooking is a snapshot of existing practice in American cookery. Because the "Joy" is recording trends, rather than breaking ground, it's an indicator of what household cooks were doing around that time. For example, the 1975 edition uses a lot of canned and packaged ingredients, and it hints at the beginning of the "ethnic" cooking trend (although the recipe for "refried beans" seems to have been created by somebody unfamiliar with Mexico). By 1997, the packaged ingredients are mostly eliminated (aside from "traditional" canned goods like stewed tomatoes) in favour of fresh ingredients.
The Joy of Cooking documents social trends. Aside from the "ethnic foods" I've already mentioned, it's possible to observe how broader social trends and current events affect the "Joy". In 1964, when writing About Water, there are instructions for purifying your water and an admonition to not use water that has been exposed to radioactive fallout. This section has disappeared from the 1975 edition, but the section on mixing cocktails survived from '64 to '75, only to be cut from the 1997 edition. Interestingly enough, the 1975 edition introduces a description of how to tap your own maple trees, which disappeared in the next edition. Unless this is a quirky nod to the hippie back-to-the-earth movement of the late '60s and early '70s, it's an odd thing to include.
For more information about the social history of 20th C. American cooking, see
Shapiro, Laura. 2004. Something from the oven: Reinventing dinner in 1950's America. New York, N.Y.: Viking.
which does a wonderful job of explaining the hideous food my mother cooked for me when I was a child.
Just to show that the usefulness of handbooks as historical source documents is not limited to the domestic world,, I will never throw out my other favourite handbook, the 13th ed. of The Chicago manual of style, since it's the edition of that illustrious orange bible that has an entire chapter devoted to describing the printing process, both letterpress and the newer lithographic printing process.