Laid-back former Montana cop Leo Banks saunters into his hardcover debut with a visit from old flame Sarah Heyman and her husband Gerry, Leo's onetime best friend, who come calling with an urgent request: Can Leo help Gerry investigate the 50-year-old drowning of Grace Turk in order to help him vindicate retarded Mickey Cochran, the convicted suspect who's sworn his innocence just before getting released from 50 years in stir to die of cancer? Naturally, Leo says no, and, naturally, Gerry shortly after gets killed. So Leo's off to Mauvaisterre, Illinois, to tilt with the local gendarmes (a troglodyte deputy, his sharp boss, and a slick state's attorney) and, more fitfully, with the suspects--especially Grace Turk's scoundrelly widower and their worthless son. The obligatory tender passages with Sarah pale besides Leo's brief, scorching scene with her own son. Good local color and fine, edgy dialogue enliven a fair-to- middling mystery.