I know people confuse Prime Reading with the reading subscription program Kindle Unlimited. But KU has a small monthly fee. Prime Reading is now part of what you automatically receive with your Prime membership. It's more like the Kindle Owners Lending Library, except the KOLL has a limit on the numbers of books you can borrow, and this does not.
Okay, I probably just confused you further. If you have Prime, and have not read ASK HIM WHY, go get it. It's absolutely free to you.