easy

This is needed both for courage and hardcover books.
Common Wrong Answers
“Backbone”
While 'backbone' is related to courage metaphorically, it does not refer to books in the same way as 'spine' does, which specifically describes the physical part of a book.
“Cover”
Although 'cover' is a part of hardcover books, it does not relate to courage. 'Spine' is the term that connects both courage and books.
“Heart”
Though 'heart' can symbolize courage, it has no connection to books, making it an incorrect answer in the context of this riddle.