I'd beware of Kindle Unlimited. You get access to only some books. It's something you can use and get value out of, but it's not something to get if you want to target specific books.
Audible has a good bit of "free" content as well. It's not all al a carte. To add on to what I had said earlier, I have four credits now and will get two move in a couple of weeks. Right now, they are having a $5 sale on a a hundred or so books, and there are a number of good ones there. Obviously, you aren't going to be reading it if you were to get into Audible. But I just wanted to give you a reference to the throughput for books that I get. I spend at least 100 hours a month listening to books, so the value is a no-brainer for me. But if you prefer reading and don't care about a limited selection, you may have a good deal there.