Lately I've heard from a couple of fans who thought they had to own a Nook, Kindle, or similar device to download and read eBooks. Not So!!
Dreamspinner Press, Smashwords, and other sites, offer their eBooks in several formats, and most of them allow you to download them more than once in different formats.
All you need is to download an eBook in PDF format, and you can easily read it on your computer.
NOTE: As far as I know, Amazon and Barnes & Noble only offer Kindle and Nook versions respectively.