I just started doing this with my photo books and I love it. Did you know that you can print on the spine of the book, or even put a photo on the spine of the book? I love being able to put a title on my book that I'm able to see without having to pull my books off the shelf.
First a quick caveat: you can only print on the spines of Custom Path books with hard photo covers or padded photo covers. Sorry, no Simple Path. And sorry, no leather covers.
Here's how you do it:
When you're editing a Custom Path book, click on the "Cover" tab in the top middle of the page. Then, make sure that either "Hard photo cover" or "Padded photo cover" is selected from the "Cover type" drop down menu that shows up in the top left-hand corner of the page. In the editing window on the right, you should now see an image of the front cover, back cover, and spine of the book. Click on the spine of the book and you should be able to edit the text box there - or you can click on the spine and then use the tabs on the left of the screen to choose the layout that lets you put a photo on the spine, or a background with a contrasting color and texture.
Hope you enjoy this as much as I do!