There are also author-owned stores such as
Closed Circle. In other words, e-book sites owned by print authors who use these stores to sell their books and the books of other print authors. So far, most of those offer only DRM-sites. It's a fairly new category -- and one that I hope grows.
There are also e-book-only publishers such as Samhain, Elloras Cave, Double Dragon Publishing (DDP). etc. Yes, most of them specialize in erotic romance, but many of them sell other types of books as well. (For example, Samhain sells young adult mystery, urban fantasy, etc. -- just read those content warnings. ;d) Others, such as Double Dragon, sell e-books in a variety of categories, including SF, fantasy, etc. Cerridwen was established by Elloras Cave to offer non-erotic romances as well as other genres. Through
Page Turners, Renaissance Ebooks sells mysteries, adventure, etc., including lots of classic pulp fiction reprints.
There is a previous thread on author-owned ebook stores here:
https://www.mobileread.com/forums/showthread.php?t=75552
These were started by established print authors -- some SF/fantasy authors, some mystery authors, some romance authors. So far, most authors are selling only their older out-of-print titles (because that's all they have the digital rights to). However, in many cases, those are their best books or at least the ones most in demand because they're out of print.
I should mention
Macabre Ink Digital because 1) I didn't link to any horror publishers earlier and 2) David Niall Wilson is one of our members.