Double Negative Once Removed

Holy cow--this one will keep Alzheimer's away, at least for a while. What a twisty, turny concept, only to be turned on its head yet again at the end. The premise is that four strangers overhear a scream while they're at the library, forge friendships based on that experience, then begin to look askance at each other as the story progresses for predictable reasons. But, there's more to the story, and it's definitely not predictable at all.