I created an account and a book, found it in my Dropbox and I edited the Book.txt file. I then returned to the Leanpub website and generated a preview. However, the preview didn't have the contents of Book.txt that I just updated. What's up?

You need to write your actual content in separate plain text files written with Markdown formatting. The Book.txt file is just a list of files. You can see the sample content for each Leanpub book styles by clicking on the following download links for business, fiction and technical books.