[JMS]

Definitely Amazon Kindle it’s the biggest outlet. You can also use Smashwords to get books out in epub like iBooks. I’ve done it and it’s really pretty easy to get your book out there.

[jcon40]

so have you sold any yet?
How long has it been available?
Did you have a cover done by outside artists?
Did you do some of your own marketing too? What type book ? Do i have to buy it if you answer all my questions?

[tbw2]

I agree with Amazon Kindle. You have a large market, and there is nothing but your time invested up front if you go for free - but you can pay to use their marketing services.
I have several books on Amazon both in print and Amazon Kindle.