Just to be clear before you go any further, please not that we do not require ISBNs! You can publish a book on Leanpub without an ISBN.
However, we do have a feature that lets you insert an ISBN into your book, if you are using one of Leanpub's own writing modes (in other words, this feature is not available for books using our "Bring Your Own Book" writing mode).
The ISBN will appear on the colophon page, which comes after the book cover, and before the table of contents.
To add an ISBN to your book, go here:
...making sure to replace YOUR_BOOK with your book's unique address.
You can also find a link to this page on the Overview page for your book:
When you go to the page, you will see something like this:
You can add a separate ISBN for each format.
After you enter an ISBN, be sure to click the blue "Update Book" button at the bottom of the page.
When you next generate a version of your book, you will see the ISBN on the colophon page.
Just to be clear, we do not require ISBNs!
Again, please note that this feature is not available if you are using our Bring Your Own Book writing mode. These ISBNs are added to books that written using one of Leanpub's writing modes; we don't add anything to the files you upload when you use our Bring Your Own book writing mode.