From what I have been reading in the community forums for the KDP (Kindle Direct Publishing) site, there is a ton of junk out there.. I guess a lot of people will cut and paste some stuff from google and try to publish it and make money. Some even come on the forums there and complain that Amazon is screwing them when their stuff isn't selling.
The sheer number of authors blows me away. My first day sales (units actually purchased, doesn't include the Prime downloads or Kindle Unlimited people) was 10 units. That put me at a sales ranking of about 20,000. The next day my unit sales were 3, and I dropped to #47,000. That means that there are over 27,000 authors who are selling between 4 and 9 units each day.. Keep in mind, in that range the typical author is making around $3-10k per year if those sales hold up. And only God knows how many authors are out there with less than 3 sales, or aren't selling at all. If the number of authors complaining on the forums are any sign, it is a ridiculous amount. I did 8 sold units (and 6 of the others which I still get paid for) yesterday, and hit 23,000 in the rankings.
Aside from the nook, I don't know of a device you can't download a Kindle Reader program for. I read my Kindle books on my computer, my android phone, my kindle hd, and my kindle paperwhite. Mostly on my Android phone because I have it with me when I get stuck in a line or waiting for something. I prefer the paperwhite at home, but when I travel I usually take my kindle hd so I can watch movies and stuff too. Amazon has made it pretty darn easy to buy and read their ePubs, even if you don't want to buy their hardware.
One successful author said his sales were 60/40 between Amazon and Nook, with nothing appreciable in any other format. Oh, and PBJ mentioned goodreads.. Amazon just bought them, so I am sure that soon my book will end up there as well, lol. After the 90 day Select program I am in now, I will switch to a different site that publishes to 5 major ebook sites. Until then I have to be exclusive to Amazon, I can't even give away copies to friends or so much as post the first chapter on my website.. It is worth it though, I have nearly 20 sales through the KOLL, KU program (in five days) and I get royalties for them too, just not quite as much as a regular unit sale.
Hoping to hit the 50 total sales mark today, I am at 47 right now woot woot!