They fold up so small, those letters...

From LitHub, a tale of when the watchers need watching themselves:

...This is a microcosm of the danger facing American archives. Because almost nothing is catalogued at the item-level, most of the unique material housed in these most important of repositories is particularly vulnerable to theft. When someone like Breithaupt steals a book, even a very old book, there is a catalog record that tells us it is missing—and likely some kind of duplicate copy somewhere else in the world. But when he steals a letter from Flannery O’Connor to John Crowe Ransom—unless that letter has been photocopied by another person—it basically ceases to exist. Not only do we not have the information in it, but we don’t even know that we don’t have the information in it.
Because the story of this county is written on the backs of single sheets of paper, the theft of archival material is nothing short of a disaster. 

Books, articles, long-form journalism, documentaries—all of our nonfiction, and a fair amount of our fiction relies in some way on what is in these archives. It is a fact drilled into students, and held sacrosanct by professional writers and historians, that primary research is the gold standard. The best nonfiction work is done not in wholesale quoting or paraphrasing of prior books, but in the steady accumulation of a narrative from single sheets of paper. The loss of even one or two of these can leave a gap in our collective memory..."