I may have read this novel/novella in the ’80s or ’90s, not sure exactly. The setting was mostly in a near-future Connecticut, but not entirely.
The premise was that someone had invented a way to “delete” chunks of reality using a gizmo no more complicated than a pocket calculator; when you deleted a chunk of reality, it didn’t magically adjust physical records of its existence, but nobody would remember it. It was just as easy to undelete the chunk of reality, which would also restore people’s memories of it - but only the gizmo that deleted a chunk could undelete it.
The story was mostly about a civil war (Beirut-style, not US-1860-style) or terrorism conducted with these gizmos.