To create a sample book if you are writing in plain text in Leanpub-Flavored Markdown, create a file in your manuscript  folder called Sample.txt . Then, type the names of the files you want included in the sample, just like you do in Book.txt . The next time you publish a version of your book, the sample will appear for download on the book's landing page, to the left of the book's cover image.

If you are using Markua, instead of Leanpub-Flavored Markdown, here are the instructions:

http://help.leanpub.com/author-help/how-do-i-make-a-sample-book-available-when-im-writing-in-markua

If you are writing your book in plain text using Dropbox or GitHub, you can check to see if you are using Markua or Leanpub-Flavored Markdown by  going here in the "breadcrumb" menu:

Author > Books > [Your Book Title] > Writing > Markdown Dialect