How to translate the book pages content using the Book Translation module?
If I understood your question correctly, you need to enable translations for the "Book page" content type.


Go to /admin/structure/types and click on "Edit" next to the content type you use for storing the book pages.
Under "Publishing options" make sure that "Multilingual support" is set to "Enabled, with translation".
Save the content type.
After you save this, you can go back to the edit page and configure extended language options in "Multilingual settings" tab.