Why does the "plus" not refer to books? The translation suggests it refers to time, ie "no longer has" rather than "has no more books".
I believe (I'm not 100% sure) it's because you are essentially adding a word that isn't there. You've "used up," so to speak, the "ne...plus" when you wrote "no longer." So to have the "any" in addition to that, you need to have a word for it, e.g. "Elle n'a plus aucuns livres."
aucun livre has to be singular.
The only exception is when the noun only exists in plural, like "aucuns frais" (expenses/costs).
"she non longer has any books" = elle n'a plus aucun livre.