Everybody has already heard of Kindle Unlimited, so no need to bother with that one. Many are unaware that it is (1) not the only such service, (2) not the best deal for your money and/or (3) offers almost no professionally published books. It all depends on what you want, from professional authors and publishers, independent authors, those inbetween and (yes) whether you get to keep those ebooks.
Did that last point catch your attention? Unfortunately, the one I was thinking about is currently having problems, so more on this in a later installment.