Bit of a late reply to this thread, but I've been digging into the Book Module's custom layout capabilities while stuck at home...
From what I have been able to test, you should be able to change or even delete and replace the photo and text objects in any predefined page template - including the cover.
If you want to clear out some or all of the predefined objects, just right click on a cell and select "Remove Selected Cell" - all the way to a blank page if you want.
Then you can add new Photo Cells (which come with optional Photo Text objects) and Photo Descriptions (free text or metadata based title, caption, or filename linked the corresponding Photo Cell). These can be repositioned or resized freely on the page (or 2 page spread)
On a cover or any other page, you can also:
- show or hide Page Text
- optionally add a background color and/or image (the front and back cover appear to be treated as a 2 page spread)
- and Photo Cells can span the gutter of a 2 page spread (use caution if you are doing that with a text cell or you'll lose text in the gutter).
That said, I've not tried any of this for the portion of the cover that covers the spine of the book.
So, while you still may want to go out to Photoshop or InDesign as your preference or to have even more control, you can do more in the Book Module than is obvious.
Hope this helps - and if any of this is inaccurate, please let me know. Documentation is not the Book Module's strong point!
Don