There’s a Facebook group dedicated to letting you know about free books from Logos and its affiliated FaithLife companies.
Some of the books are old, out-of-copyright books by long-forgotten authors. But plenty are books on my wishlist. So pay attention and get free stuff — if you are a Logos user. And if you pick up a few books that are part of a set, if you later buy the set, you get credit for already owning part of the set (even though you paid nothing for it).