The first quarter of An Ace up my Sleeve is absolutely excellent. Straightforward, adult and gripping with a great twist. The second half of the book meanders a bit, with some clever cat and mouse, back and forth between antagonists, though it is never dull and you definitely want to find out what happens. Helga Rolfe is a beautiful middle-aged woman, married to a super old businessman. She is lonely and horny as hell (this is portrayed as her one big flaw) and picks up a studly American while traveling alone to Switzerland to meet her husband at their sick cliffside mansion. He is a big strong naif, AWOL from the army and got rolled by a woman who picked him up at the bar. He's weirdly competent, though, and seems to constantly avoid getting into a situation where they could hook up. Then things get interesting.
I'm going to stop here but just say the ending is really interesting on a sociological level, though surprisingly soft given JHC's brutality in past books. I learned afterwards that there are two other novels continuing Helga Rolfe's adventures. I will be looking for them and let's hope she gets laid!