I have used Microsoft Word and Leanpub[1]. Super easy and I like the set your own price.
There are several ways to write your ebook: For instance, I have written mine simply by using markdown, uploading it to GitHub [1] and giving Leanpub access to it as collaborator. That way Leanpub is able to pull the recent manuscript from your repository for a new release or generating a preview. In addition, hosting the manuscript on a public GitHub repository (in case your ebook is open source), gives you the ability to make use of collaborating with others on it. Highly recommended platform!