Thanks for your comments on the book, hilmi!
There is no literary_piece.txt in the source code, because what we are showing in the Create the literary_piece content type section is the fact that you can export any content type and save the result in a text file, and then import the contents of that text file as a content type.
Since the source code is a Drupal instance which has the literary_piece content type already created, you can export that content type and paste the results in a text file yourself.
So: by design!
Hope that helps, thanks so much for communicating with us here, and please continue to reach us with your questions and concerns!