Unexplained laughter - Alice Thomas Ellis

Behaving badly made Lydia feel better. She hoped she wasn't turning into one of those maniacs who murder people in order to establish their superiority over their fellows who say Please and Thank you and conform to the basic customs of society.

Recovering from a love affair gone wrong, Lydia retreats to the Welsh countryside, leaving behind her sophisticated friends, but accidentally inviting Betty, "the human equivalent of sackcloth and ashes," as her companion. There they encounter Hywel, a dour farmer, Elizabeth, his nervous wife, the aspiring priest Beuno, Hywel's brother, and randy Doctor Wyn. Meanwhile Hywel's strange sister Angharad roams the land, observing all, while Lydia is increasingly unnerved by the unexplained laughter that comes down from the hills.