From the authors of the "excellent" Mr. and Mrs. North Mysteries: Captain Heimrich's vacation in Key West turns stormy when a Communist snitch is silenced (TheNew Yorker).   Capt. M. L. Heimrich of the New York State Police may not have the flash of hard-boiled city detectives, but there's no lead the intrepid investigator won't follow until his every hunch is satisfied . . .   There's no better place than Key West for relaxing under a palm tree with a daiquiri in hand. It should be the perfect location for Captain Heimrich to recover after being shot in the shoulder during a confrontation with a killer. But Heimrich's recuperation soon turns into a working vacation when a fellow guest at the Coral Isles is murdered.   A former Communist famous for exposing his compatriots, Bronson Wells wasn't much liked--giving the local authorities a long list of suspects to sort through. And even though Captain Heimrich is outside his jurisdiction, the experienced detective can't resist doing some investigating of his own.   With the assistance of his associate, Dr. Barclay MacDonald, and Miss Mary Wister, a young artist from New York, Heimrich will forgo the sand and sun of the Keys and digs into possible motives for wanting the arrogant Wells silenced forever.   Death by Association is the 5th book in the Captain Heimrich Mysteries, but you may enjoy reading the series in any order.