Click on the left-hand corner, then go to books and audible, then to prime reading. There is a selection of books that can be read for free

If you mean Kindle e-books, you need the Kindle app, it takes a bit of getting used to, but there are thousands of free books and magazines with Prime, there are various places to click to see a selection, it rotates every few months. Usually there's a button that says "Free with Prime" somewhere on the page, and that narrows the display to just the Prime stuff.

You will go to the section that says "Books and Audible", then you will choose the category that is titled "Prime Reading". Under the "Prime Reading" category, then you will be shown a giant selection of nothing but Prime-eligible books/audibles. This Prime selection is your option that includes all FREE and Prime eligible books and audibles. This is where you will be able to search for what you're looking for or what genre best suits your mood in which you are wanting to listen to and/or read yourself! I hope this helps! (: Great question!