One by One

More psychothriller than mystery: the question isn't so much who as why; and, I suppose how, in the sense of, how did they expect to get away with it. It kept me entertained enough on a cold drizzly morning, but I'm unlikely to remember much about it in a month.

The real burning mystery is: why did I read this? Someone recommended it to me, months ago, and I placed it on hold at the library – but without logging my usual side notes. Whodunnit? And why?