More observations from my adventures in independent publishing:
(Obligatory plug: you can buy both Forever Changes and It Takes a Worried Man on Kindle, Nook, and Kobo, secure in the knowledge that 70% of the bargain price goes straight into my pocket. Two months later. You can also buy many of my other books in those formats as well, but those are the only two that I'm doing myself.)
There are pretty powerful incentives to publish exclusively with Amazon right now. If you enroll your book in their exclusive program, you can have it available to read for free in their "library," where you will actually get paid if someone borrows it for free. Sweet deal! And just judging from my own sales info, Kindle readers outnumber Nook readers by about 4-to-1. Then again, judging by my sales data, Kobo readers do not exist. And I know they're out there. I have seen them in the wild. Twice!
So, yeah, Amazon is pretty okay for a short-term fling. But that exclusive deal is, I am convinced, going to bite you in the ass eventually.
With most ereader hardware being more or less the same at this point, one of the things people are going to look for is the book selection. Having your books available on Nook and Kobo will help convince more people to buy those. And if some writers are signing up for Amazon exclusively, you'll build goodwill with the Nook and Kobo buyers by being on their stores. And you never know which of those buyers is going to be one of those magically influential people who gets all their friends to buy a book.
(Prediciton that you can feel free to mock me mercilessly for if I'm wrong: I think Kobo is going to overtake Nook. They've just signed a deal with the independent bookstores, thus positioning themselves brilliantly as the independent alternative to a Kindle. Which leaves the Nook kind of hanging in the middle as the "representative of a big (but not as big as Amazon) corporation" model.)
Competition among ereader manufacturers and ebook stores benefits writers, and going for the short term gain might have a bad long-term impact. If Amazon crushes B&N and Kobo, how long do you think that 70% royalty is going to last? A week? Two? Once they've cornered the market, they can start squeezing their content providers. (I don't think this is unique to Amazon. Any corporation will do what it can to maximize profits.)
The current boom in independent digital publishing is fueled by the competition between these companies. If any one of them "wins," those of us providing content for them will lose.
There's no question that you can make more money in the short term by signing up with Amazon exclusively. But I think putting your book on multiple platforms is a smart investment in continuing earnings.