You may know that Amazon has a program called Kindle Unlimited which costs $10 per month.
It allows you to borrow any books that are in the Kindle library (which is a lot!)
But as a Prime member, you can use the Kindle Lending Library without joining Kindle Unlimited, and you are entitled to one free book download a month.
There’s also a program called Kindle First, which gives Prime members access to books that have not been generally released yet. They feature six books each month, and you can choose to download one of them.