We actually don't have a "sample book" feature for unpublished books, because our philosophy is to "Publish Early, Publish Often".
This means that if an author has already written some chapters, we believe they should go ahead and publish their book already, adding chapters to the published book as they write them.
But of course, there is a workaround...
People who have some chapters they're willing to make public, but still aren't ready to hit the Publish button on their book, have sometimes copied and pasted the text of a chapter or two into the bottom of the About the Book section for their book. That way, people can easily see the chapter or chapters, and they don't even need to download anything.