Amazon is biggest because they were early, and someone has to blaze the trails. But I don't think one can say they have a monopoly any more, not for the last year. Like with everything else, when enough people start to have the devise, content providers will flourish.
I keep discovering new online stores. Interestingly I've found that even for stores specialising in adult stuff, Amazon will have all the same and often cheaper. So I'm not really sure what type of stuff you mean that they don't have a variety of?
My only complaint is that a lot of books on Kindle are only available in US. But that is the publishers deal, not Amazon.
Over here a lot of people using readers with e-pub are frustrated because book stores in UK who sell them, have stopped selling outside UK. Publishers again... They seem to be as short-sighted as the music corporations.