Here Comes a Chopper

A VINTAGE MURDER MYSTERY
Rediscover Gladys Mitchell – one of the 'Big Three' female crime fiction writers alongside Agatha Christie and Dorothy L. Sayers.
When a pair of ramblers, lost in the English countryside, stop at a country house to ask for directions, they are most astonished to be ushered in to dinner. It seems they've been invited as a necessity by the superstitious lady of the house, to avoid thirteen guests sitting down to dinner. But the thirteenth guest never arrives, and his headless body is discovered in a wood the next day.
Fortunately, numbered among the original dinner guests is a rather extraordinary psychoanalyst, and sometime detective, by the name of Mrs Bradley...