Today I happened to become aware of a website called "Puritan Books." (link to site) I like the basic concept the web site owner has, of selling .pdf versions of old/rare books. I think he may run into some difficulties, because of (a) the fact that he cannot copyright his work (all the books being in the public domain); and (b) the fact that Google Books (and Archive.org, and the French national library, and the University of Michigan library) has an ever-expanding collection of books with downloadable .pdf versions.
As a caveat, I have not done business with this website. If any of my readers have, I'd love to hear about your experience.