spine of a book
Refers specifically to the part of a book cover that faces outward when shelved, often bearing the title and author.
本棚に並んだ背表紙を見て、読みたい本を探す。
I look at the spines lined up on the bookshelf to find a book I want to read.
Compound of 背 (se, 'back') and 表紙 (hyōshi, 'cover'), referring to the back part of a book cover.