Dark of the Moon — John Dickson Carr

When Dr. Gideon Fell, that most eminent of eccentric sleuths, finds himself at a party whose guests are in a state of deep agitation, all the faculties of his detective genius are called into play.

Why is the host of the party -- Henry Maynard, a Southern Aristocrat -- so cryptic about the strange goings-on in the stately mansion? And how is the theft of the scarecrow linked to a diabolical and ingenious murder? Fell's cunning proceeds to uncover the dastardly motives to the deeds that have been taking place in the dark of the moon.