Pro Tip: Click here to get Kindle Unlimited from Amazon if you're an avid reader who doesn't have a Prime Membership. If you're already subscribed to Amazon Prime, then use Amazon Reading for convenience, here.
Prime Reading is a benefit of your Amazon Prime membership. It offers many reading services. It includes 1,000 Kindle books, magazines, and comic books. Kindle Unlimited (found on Amazon, here) is a separate subscription. It provides unlimited access to 1.4 million+ Kindle eBooks and magazines.
Prime Reading gives you access to Kindle's premium eBooks, but you need an Amazon Prime membership. Prime membership costs $12.99 per month ($199 per year). This membership also includes many other benefits, including Amazon Prime Video.
Kindle Unlimited is a book worm's dream come true. It costs $9.99 a month for your subscription. You get an unlimited amount of access to all the eBooks and 2000+ Audible books. For avid readers, Kindle Unlimited is the best option.
However, if you've already got a Prime Membership (get yours from Amazon here) then it's cheaper to use Prime Reading.
To get the best value for your money, check out Amazon prices, here. Save big and get the best prices, with a variety of items to choose from.