Sample books are dealt with differently in Markua than they are in Leanpub-Flavored Markdown.

Basically, you determine the sample book content in the manuscript itself, and not in a separate Sample.txt file.

Before a section that you want in the sample, you type {sample: true}  on a line by itself, and then you type {sample: false}  where you want the content after that to be excluded from the sample book.

If you go here https://leanpub.com/markua/read and search for "sample book" and scroll down a bit you'll find the following example:

Please note that in your sample book, people will be able to see all of your section headings, though they will not see the actual contents of sections that are not included in the sample book.

The idea here is that it's useful to readers to see section headings for the whole book, as it will give them a sense of how much content is in the book, and encourage them to buy it.