Please note that most Leanpub readers are quite understanding when a book project is started and stopped in good faith!
If you are going to abandon a book project with no sales, we recommend you back up all your work (in case you change your mind!), and then "close" your book:
If you are going to abandon a book project with sales, we recommend that you "retire" the book here:
(Please note that Leanpub customers can get refunds easily within 45 days of purchase, but we do not issue refunds for purchases older than 45 days.)
Then, you can update the About the Book section with an explanation here:
If you want to, you can put e.g. your email address in there, and ask people to contact you, if you would like to issue refunds to them individually from your own funds, if/when they discover the book has been abandoned.