The phrase "book spine" refers to the vertical part of a book's cover that faces outward when placed on a shelf. It usually includes the title of the book and the author's name in order to help identify and locate the book easily. Full definition
I love the little book spine in the «k» — LOVE it. (sarahbessey.com)
It is important that you or your cover designer know the correct dimensions in order to make sure that your printed book spine is ready to standout. (adazing.com)
This plastic hard case features a nostalgic photo of book spines of popular children's books. (ebookfriendly.com)