@skaeth@mark@bookstodon@sffbookclub@fantasy Mostly here on Mastodon, honestly. I also get some recommendations from assorted other websites I follow, none of which are focused on books, but they have some conversation about arbitrary topics which occasionally includes books.
I also get a fair number of them from the free book deals on Barnes & Noble either "Free Friday" or the one free ebook per month that members get (although now that's restricted to only premium members, so a smaller audience). I will say, the fact that I can download a free sample from B&N and read it is a huge benefit; there is no better way to determine whether I'm likely to enjoy a book than getting to read part of it. So I almost never actually buy a book these days without reading at least a sample of it.