I had a stint where I bought and tried to read a few dozen cheap $8-$14 Dover math books. There is a reason most of these books were not revised and republished before their copyright expired and they became Doverable, and it is not because they were perfect originally. Poor 1940s typesetting, only the barest sketches of images, and linguistic quirks that are not present in modern language. The recommended booms from my high school and college courses were much better. Of course, 90% of everything is crap, so there logged be a few gems in there.

The $3 copies of Plato et al were better than the math books.