Over the Christmas holidays I read two Agatha Christies, Five Little Pigs and Black Coffee, the latter being a novelisation of her first play by Charles Osborne.
The former, unusually, was too obvious in solution (hard to explain without giving it away, but the passivity at trial clearly points to one thing, which to a seasoned reader of crime fiction then points to something else) - which isn't necessarily a big deal, and I don't tend to make much effort to try to work out whodunnit in general, but there was little else to the story but that.
The intro to Black Coffee indicates that the play had initially been turned down for production, before being successfully produced. It is fairly insubstantial - it would be worthy of a short story, really, rather than a full novel, I think. One for the completists (as, I guess, would probably be all the plays, most of which haven't been novelised).