I came across this the other day:

https://github.com/vhf/free-programming-books/blob/master/free-programming-books.md

A Github repository of various free programming books. Looks like it covers a wide variety of topics, and is regulary updated. Some of the books require registration on a site, but that’s the purpose of a burner account if you want. Most are either HTML or PDF, but I’ve found a number of them in EPUB or MOBI format as well.