If you enter a bookshop and you ask for a book that does not exist then it's definitely your mistake.

If you ask for a book they don't have it's a different matter.

In any case, when you ask for a book in a library you are using their "search" endpoint. The equivalent to opening a books/1 url would be asking for a specific instance of a book by serial number or so. Then it's clear that you made a mistake uf you do that for an unexistent serial number...