Twelve books into a series of mysteries set in England in the aftermath of World War I, the mother and son who team-write under the name of Charles Todd keep finding new ways to gauge the emotional effects of war on the living and the half-dead. The story behind THE RED DOOR (Morrow/HarperCollins, $24.99) is so sad, the book should come with a warning sticker. It begins in Hobson, a village in Lancashire, on the day after the Armistice is signed -- at the 11th hour on the 11th day of the 11th month of the year 1918 -- when Florence Teller hits on a suitable gesture to welcome her husband home from the war, painting the gray front door of their house ''a vibrant and glorious red.'' Two years later, Inspector Ian Rutledge of Scotland Yard arrives at that same front door, now faded to a dull rose, to investigate Florence's murder. Sensitized by his traumatic experiences in battle, this compassionate detective makes his way through the house, gathering evidence of a solitary life defined by constant loneliness and sudden, sharp loss: of the beloved son who died young, the aged aunt, the pets buried in the garden. Yet none of these losses were more keenly felt than that of Peter Teller, the soldier-husband who rarely came home between postings and vanished after the Armistice. [ABSTRACT FROM PUBLISHER]