All I can tell you is that if I want a book (I'm in the U.S.), if it is in ebook form 99.7% of the time I can get it at Amazon. No muss, no fuss and for a great price.
While GA Russell is right about the relationship with Amazon, I personally have no problems with it. They've done right by me more often than not.
I'm looking to buy my next reader from another company but I will always have a Kindle (I have 2 now!).