Message on Google Play's website: We’re currently accepting a limited number of new publishers to the Google Play Books Partner Programme. You may express interest in joining the programme through our online interest form. Please bear in mind your form may not get accepted, just keep trying.
Already a partner? Click here to switch to the account associated with your Partner Centre. This means if you have a Gmail (Google email) account you should be able to just sign into Partner Centre. This is the best way to join them and start publighing your books.
Here is also a link to publishdrive.com 6 Facts You Didn’t Know About Publishing on Google Play https://publishdrive.com/6-facts-didnt-know-publishing-google-play/