The incidents over this past year — including a missing book that was discovered only this week — were not the first time books have mysteriously disappeared. Thirty years ago, the library lost so many books on human rights to theft that they had to be placed in a locked cabinet. The latest works to be targeted cover a wide range of topics, from gun control and women’s suffrage to LGBTQ issues and how people of color fare in the criminal justice system. About half the books specifically deal with President Trump.

While none of the books in the latest incidents appear to have been stolen, some have been hidden in ways that made it nearly impossible to find them when patrons wanted to check them out. They have been discovered inexplicably filed in the wrong sections, hidden behind a row of Stuart Woods novels, or shelved with the spine facing inward.

Ms. Ammon said she and other workers at the library have hunches about who might be hiding the books, but they have yet to catch anyone in the act. The perpetrator has some support in the community: After a local television station did a story about the missing books, one person called Ms. Ammon to praise whomever had hidden them, complaining that the library only carries books that represent a liberal point of view.

Ms. Ammon said she asked the caller to provide a list of books that should be in the stacks, and while the person failed to provide one, she suspects the library already has whatever might be on it.