With her sense of "abandon and play," Langton sends her scholar/sleuth Homer Kelly up the down staircase on a labyrinthine search for a missing art lover (Eudora Welty).   Leonard Sheldrake knows little about Frieda except that he loves her. A Harvard professor and admirer of the bizarre engravings of M. C. Escher, Leonard is visiting a Cambridge exhibition of the artist's work when he meets Frieda and falls instantly in love. As they trade remarks about the artwork, he learns a few brief things about her. Though young, she is a widow, an orphan, and has a terrible secret in her past. It is only after she vanishes that he realizes he didn't even learn her last name.   Leonard enlists fellow professor Homer Kelly, the amateur sleuth, to help find this beguiling young widow. But as they comb Cambridge for the woman in the green coat, Homer and his friend find themselves slipping into a mysterious labyrinth, whose treacherous dimensions are as impossible to grasp as anything dreamed up by the late, great M. C. Escher himself.