I don’t know how many have noticed the availability of this feature.
For $9.99 a month it gives you access to around 700,000 books. It works like a lending library you may have 10 books on your reader at a time from Kindle Unlimited. If you want a 11th book it will pop up and ask if you wish to return your oldest Unlimited book or give you the option to select another book to return.
Being a rabid reader I find this to be a lot cheaper then buying books from Kindle and then realizing they are crap. This way I can try before I buy just as I do with the local library. Or if I’m researching something I can get it and use the reference as long as I wish.
A few titles that may be of interest are.