Reply To: how to auto fit and scale background for book spine?

Step by step:

1. Create new document with 3 facing pages;
2. In the hamburger menu on the pages panel deselect ‘Allow Document Pages to Shuffle’;
3. Move the 1st page to snap with the other two to form the cover pages;
4. Select the ‘Page Tool’ (3rd from the top);
5. Select the spine page;
6. Set the left and right margins to 0 (or any other small value);
7. Now resize the width of the spine and the right most page (the front page) will snap to the spine.