If it's confusing for authors, imagine what it can be like for a reader. There is the basic decision assistance tool--the library. But they may not have a copy (paper or ebook) when you want it or it may be a book you want to buy. If you buy books to keep forever, hardbacks or paperbacks are often the choice. As one who moves a lot, I lean toward ebooks when I buy. When I want to hold paper, I go to the library, which is often.
I have a Kindle Fire and a Nook HD, so there is an additional choice of the format for ebooks I buy. The Kindle is smaller, but the Nook HD (for all its larger size) is much lighter and the on-screen keyboard is larger. That's important when I travel and want to access the Internet on my ereader. However, I sell way more books on Kindle than Nook, so I usually buy more for the Kindle.
Back to author marketing. The online sites I push are Amazon, Barnes and Noble (BN), Kobo, and Smashwords. Kobo is big in the Canadian market, and Smashwords translates a book into all ebook formats. That's a big plus for authors. You can download for any device, including in a pdf or text file if you read on the computer. (This is me being crafty. Each link in this paragraph is for a different book.)
Readers seem less aware of Smashwords than authors are. Smashwords loads books to all sites except Amazon. However, they format books in a mobi file (for Kindle), so you can buy books for Kindle there. Smashwords is a way to put my books on itunes (iPad, iPod) and smaller sales sites, such as Kobo or Sony. I load them to Amazon and Barnes and Noble myself. An author earns more and is paid faster by selling directly through a site. Still, I bow to Smashwords. What a concept.
I have recently learned that Amazon's Kindle Boards creates a page for each of your books, and that page has a link to all of the Amazon sites. That means a reader can go to one spot to link to a book in the U.S., Canada, U.K., Germany, Mexico, and on and on. This has become an important marketing tool for me.
Appraisal for Murder http://www.kboards.com/book/?asin=B005PJM8WO