I'm probably not a good data point right now because I'm mostly reading research stuff, which comes to me by the highly technical qualification of: materiel on my topic of interest that the library has, as found by tag searches on the library website.

However, the last ten or so books I've read that weren't period research:

-Rivers of London: reread, but originally borrowed from library and then bought after I saw discussion of it on a blog about racist coverart.
-Treason's Harbour: bought after I got into the series after seeing the film adaptation.
-Wicked Gift: Bought because I read everything by that author, but first got into that author after liking her in comments over at Dear Author.
-Rise of the Spider Goddess: bought again because I read many things by that author, and started following him after he did some faminist blog posts that were linked around.
-When We Wake: borrowed from library because it was recced on a blog I read.
-Promised the Moon: borrowed from library because recced in a comic book that used it as a research source.
-Red Moon Rising: bought because daily deal on audible and it looked cool, I get the e-mails, and buy a lot of audiobooks off daily deals.
-The Black Count: borrowed from library after recced by an actor I like.
-Rat Island: borrowed from library after recced by my dad and brother.
-House at Sugar Beach: borrowed from library after recced by someone I follow on goodreads.

This probably doesn't represent that I do buy a lot of romance novels based on the goodreads if you liked this you might like that feature or goodreads recs by people I follow, and to a lesser extent sf/f novels. I get science books based on reviews in Science News magazine.