I use eBook Edit Pro -
http://www.ebookeditpro.com It's very easy to use and you build your content in HTML. What's nice is that since the ebook software is HTML based, you can put AdSense ads in your book, stat trackers, etc. It functions just like a web browser.
The downside to it is that it is not MAC compatible so you will lose that audience. These days that's becoming a bigger issue because of so many people switching to MACs.