The Moving Toyshop (Gervase Fen, 3)

Named by P.D. James as one of the best five mysteries of all time. Richard Cadogan is at loose ends in Oxford, very late at night. Charmed by the window display of an old-fashioned toyshop, he is worried to find the door unlocked; surely the owner should be alerted. And so Cadogan slips into the darkened store and up the narrow stairway to the apartment above. But rather than a snoring toyman, he finds a very dead old lady, the marks of murder still livid on her neck. But when Cadogan returns with the coppers, the toyshop...has disappeared. This, it seems, is a matter for Gervase Fen.