Thoughts
I think the best example of "things in books can be metaphors for ideas" is the ink-lickers. The ink-lickers enjoy eating rare books.
For example, one spends a lot of money to purchase one of the few surviving copies of Plato's Elements, and then chews it up and swallows it. Is this a metaphor? Yes, absolutely. It's a metaphor for rich people destroying something that has some objective public value, for sort-term personal pleasure. Is it a metaphor for any specific action or behavior or person in real life? I don't think so. The closest example might be private art collectors, but private art collectors predominantly do not destroy the artwork in their collection.