Academic publisher De Gruyter has a strange set of academic books available for free at Barnes & Noble, Google and eBooks.com. The copyright page says they're under a Creative Commons license:
You can't sell them or create derivative works, but you can give them away to others.
Unfortunately, eBooks.com doesn't allow ordering a search by price, but
Barnes &Noble does. Google Books allows setting the search to specifically return
free Google ebooks.
eBooks.com does have the books available as typeset PDFs. If you want the books in PDF and it's worth the effort to you, you can browse one of the other searches and copy-paste the ISBN into the
eBooks.com search bar. Google has PDF ("scanned pages") listed for the books, but for some reason, the ones I checked are just epub conversions like you'd get out of Calibre.
I spot-checked five at Amazon. Three were regular price, one was free and one wasn't available as an ebook.