Symbolism, Imagery, Allegory
Memorabilia is defined as something "remarkable and worthy of remembrance" (source). The Memorabilia collection of the Leibowitz Abbey doesn't stray too far from this definition either. It's a collection of books, documents, articles, and other literary mementos collected from the previous civilization to preserve for a time when humanity may be ready for it again.
The irony of the Memorabilia is that the monks of the abbey can't probably discern what should be "worthy of remembrance" and what is trivial. A tract on nuclear fission and a play about robot workers (22.83) might be housed with racing forms and shopping lists that include a pound of pastrami (2.26).
As time passes, the important becomes trivial, and the trivial becomes important. But boy, does that process take a long time.