Authors sometimes decide to stop selling their books on Leanpub, for all kinds of reasons.
For example, their book may get picked up by a publisher (congratulations!).
Sometimes this means the author is retiring an unfinished book, and they may want to make sure anyone who bought their book on Leanpub, eventually gets a copy of the finished book, when the author does complete it.
Now, obviously we are not going to provide authors with private contact information that belongs to their readers, like their readers' email address.
However, there are some things you can do.
First, check to see if anyone has opted in to share their email address with you, by going here:
...making sure to replace YOUR_BOOK with your book's unique web address.
You are permitted to contact people using the email addresses provided there, just be sure to explain everything clearly and politely.
Second, we highly recommend you publish a new version of your book on Leanpub, with a new section called "A Message to My Readers" at the beginning of the book, that explains what you are doing. In that new section, we recommend you add your email address, so anyone who wants to get a full copy of the book eventually, can contact you and give you their email address.
Third, when you publish the new version of your book to include this new "A Message to My Readers" section, we recommend you use our feature to notify your readers when you publish the new version of the book. Only readers who have opted in to this feature will get the email notification, but most Leanpub readers do opt in to this feature, particularly readers of unfinished books, who are expecting updates.
Fourth, we recommend you update the About the Book details of your book with a similar section called "A Message to My Readers", where you explain the situation, asking them to download a new copy of the book from their Leanpub library, so they can get your contact information. In this section, you may also want to explain where people can get a copy of the book when it is completed, if your book has been picked up by a publisher.
You can edit About the Book section of your book here:
Finally, you should retire your book. After you retire your book, it will no longer be for sale on Leanpub, but your book's landing page will still be visible to the public, and they can see the About the Book section. That way, any existing reader can understand the situation if they visit the page, and you can also use that page to point people to wherever else on the web they can buy your book, if it is going to be available for sale elsewhere.
Good luck in all your future efforts, and thank you for trying Leanpub!