Tied Up In Tinsel by Ngaio Marsh (Roderick Alleyn #27)
 
A Christmas pageant turns unholy.

Holed up at Hilary Bill-Tasman's manor estate for Christmas, Troy Alleyn is to paint the man's portrait and, while she's there, view the Druid Christmas pageant. Along with a pack of eccentric guests, Troy enjoys the festivities-- until one of the pageant's players mysteriously disappears into the snowy night. Did the hired help-- each a paroled murderer from the nearby prison-- have a deadly hand in this Christmas conundrum? Inspector Roderick Alleyn arrives to join his wife in finding the lost man-- and unraveling the glaring truth from the glittering tinsel.