Agatha Christie called her 'a shining light'. Have you discovered Margery Allingham, the 'true queen' of the classic murder mystery?      The late John Lafcadio's ambition to be known as the greatest painter since Rembrandt was not to be curbed by a matter as inconvenient and trifling as his own death: bequeathing twelve sealed paintings to his family, his will proclaimed that the canvasses be unveiled over the years at annual exhibitions populated by the elite of the art world.      But his quest for artistic immortality is interrupted when, during the eighth unveiling ceremony, the beau of Lafcadio's grand-daughter is stabbed to death. Albert Campion is among those present, and must untangle one of his most bizarre cases to date.