If you read a lot but have a limited budget, paying $9.99 a month for access to over one million digital books and thousands of audiobooks may be a great choice. You can keep up to 10 out at a time.
However, Amazon Prime includes the same service but the number of books and audiobooks is drastically reduced. There are only about a thousand books and few if any audiobooks. Also, content keeps changing so you may start a book and it gets removed before you finish it. Note that you can turn on Airplane mode on your kindle to avoid books disappearing while you are reading them.
Both Kindle Unlimited and Amazon Prime have a 30-day free trial so you can try one and then cancel on the 30th day and start the second 30 day free library. They also have a reduced price prime program for low-income customers.