When you work on a Leanpub book, you can click a button to create "Previews" of your manuscript, which are only visible to the author.

Then, when you want to publish your changes, you click a different button to publish the latest version of your manuscript; only published versions are visible to readers.

Here's one of our "Getting Started" walkthroughs for authors, that will hopefully make the process a bit clearer:

http://help.leanpub.com/en/articles/4828488-getting-started-creating-a-leanpub-course-in-the-in-browser-text-editor-writing-mode

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