The right thing to do here is to put the book in "Stealth Mode". That way the book will not show up on Leanpub any more, but you can still work on it.
To put a book in Stealth Mode, please follow these steps:
1. Go to where you are editing your book in Leanpub and then click on the "Admin" item in the menu at the bottom left.
2. Select "Unpublish Book" in the menu under "Admin".
3. Click the blue "Unpublish Book" button.
4. When the book is unpublished, refresh the page (or just navigate to another page and come back) and under the Admin item in the menu you will now see an option called "Stealth Mode". Click on that menu item to go to the Stealth Mode page.
5. Tick the box under "Stealth Mode" and then click the blue "Update Book" button.
Then, your book will no longer appear on Leanpub.