The Incident at Fives Castle by Clara Benson

It is Hogmanay, and Angela Marchmont is at Fives Castle, the Scottish seat of the Earl of Strathmerrick, to see in the start of 1928. But when she finds out that the Foreign Secretary, the American Ambassador and the Head of British Intelligence are also among the guests, Angela begins to suspect that something momentous is afoot. Before long, they are all snowed in and a body is discovered, and Angela soon finds suspicion directed against herself...