OReilly had DRM free books to buy. I bought a bunch.
But Piracy killed it, so now its subscription service, or available on kindle (I don't use). Some authors seem to have deals to give away their books and sell them at the same time: e.g.: https://www.tidyverse.org/learn/
I buy some DRM free books from Manning. They put a footer on each page, saying, licensed to: xxxxxxx@yyyyy.com to discourage me from sharing. It doesn't bother me.
Manning also seems to be going a sort of subscription route too. They have "live book" where you buy access to parts of books you want with tokens. Its kinda interesting how they're trying to do micropayments.