So a bunch of people have had trouble getting to the "Download EPUB" functionality with Google Play Books when trying to get their archive copy of The March North.
- If you are logged in to Google with a Google Account other than the account that bought the book, log out. (Google gets confused about whose books to show you.)
- Log in to Google with the Google Account that bought the book.
- Navigate to https://play.google.com/books
- In the left-hand vertical stack of options, click on My Books
- Next to the title of the book under the cover icon, there's a stack of three gray dots with hover text Options
- Click on the three dots; a menu will appear
- Select Download EPUB from the menu.
Updated: you don't have to hover-the-cover to get the three dots anymore, they're next to the title of the book under the cover icon.