I was surprised to find out some people thought you had to have a Kindle or Nook to read digital books, but that’s not true. Whether you have a Kindle or Nook you can generally purchase books online cheaper from Amazon or Barnes and Noble (for the Nook) than from a book store and the digital editions are usually cheaper. By the way, most of the free books are digitized books that are out of copyright protection and most of them are of very poor quality. There’s not much point in trying to save a little money downloading them; they are usually boring and sometimes hard to read; if you want cheap download the $0.99 ones.
Both the Amazon app for Kindle and the Barnes and Noble app for the Nook offer pretty much the same things. You load them on your tablet, smartphone or computer and when you buy a book once you can read it on any device with the app installed. Of course, you can also read that same book on the Kindle or Nook if you own one. I used to have the Nook, but it got destroyed when our house flooded back in May so now I just download and read books on my laptop.