I see it from the other end - what counts is not the cost of producing the book, but the opportunity cost of the reader sitting down with a particular book. A computer or phone allows you to context switch to a million different things, and even an e-reader allows you to easily switch between hundreds of books. But with a physical book, you commit yourself to carrying, holding and focusing on a particular work.

There's something deep about this commitment, and I think we would get almost the same result if we had digital devices that were made to hold exactly one book, and you had to take yours to the library/store to return the old one and download a new one - such that even if the cost of copying the bytes is zero, you pay the cost of physically carrying that one book that you took the time to pick out.

That's true too. Both costs matter.