If you owned a magic library people donated to in order to preserve media for eternity but which was going through overpopulation, what criteria would you use to decide which media survived/discarded?

I was reading a recent article about the efforts by people not to ban books. While I think the sentiment is good-natured, as a helper at my local library, this is actually very problematic. People donate to us all the time, as is how libraries work. Sometimes the books are unpopular, unproductive, harmful, or just low tier.

I would never apply this logic to human beings, all humans have value if the system knows how to channel them correctly, but books are inanimate objects where their expected purpose is to be read (if you were to say a book is useful on the basis it could be used for something like ripping the pages out for wiping a floor for example, that would make its usefulness as a book cease). Often we are over capacity from the donations, so once a year we have a book sale at the church (libraries and churches getting along? Crazy, right?), but even then, a lot just isn't sold, and we're forced to either give them to another holding place or, in the worst case scenario, cremate or trash them. I am all for free speech, but freedom to produce speech is different from freedom to preserve speech, and I'm sure even the ancient Romans produced a lot of scribbly nonsense.

Suppose you were in my shoes and the library could preserve anything forever but not everything forever. What criteria would you use in order to decide what media (books, movies, games, etc.) gets to stay and what has to go?