Here are a couple of websites, that I've run across recently, that have open access (free) ebooks:
https://www.openbookpublishers.com.
http://www.cornellopen.org.
Even though there are one or more websites which claim to list all open access websites, and one or more websites that even claim to list websites which have open access titles on particular subjects, I'm a little bit skeptical of their claims. I stiil like to peruse individual websites with open access titles, just to see what they have.
Keep in mind that open access books tend to be academic, very narrowly focused, and esoteric.