We actually don't have a concept of a "back cover", either for books written in one of our own book writing modes, or in our "Bring Your Own Book" mode.

In our "Bring Your Own Book" writing mode, the "cover" image that you upload is used for displaying the book on our website etc., but we don't actually add it to the book file you upload. So, if the book file you upload has a front cover and a back cover in it, then that's what readers will see when they download the book. (We don't do make any changes to a book file that you upload; what you upload is exactly what readers get.)

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