what metaphor does O'Brien use to describe his love for Linda?

The metaphor is death to a book.

“Well, right now,” she said, “I’m not dead. But when I am, it’s like…I don’t know, I guess it’s like being inside a book that nobody’s reading.”

“A book?” I said.

“An old one. It’s up on a library shelf, so you’re safe and everything, but the book hasn’t been checked out for a long, long time. All you can do is wait. Just hope somebody’ll pick it up and start reading.” (245)