Hard copies did appear on Amazon used, but at high prices in the $70s and way up.

I don’t know this business and can’t vouch, but they appear to be selling at a more reasonable price for anyone into a hard copy. Buyer beware, or course.
https://www.gunsitestore.com/product/library/books/principles-of-personal-defense/

Cooper is an interesting character. I would have liked to have known him, but I rather suspect that the critical mass of “curmudgeon” might have been reached when we met, and the resultant criticality might have destroyed a great deal around whatever location that was where we met.

George C. Nonte was another character from “ye olde dayes” that it would be interesting to look at. I remember looking at the Glock a long time after he wrote “Combat Handguns”, and being struck by the similarities between the two concepts. Nonte’s ideas never saw much official light, but it’s odd how many of his ideas became de rigeur in combat handgun design over the years…