A pair of inexperienced sleuths attempts to best Captain Heimrich in this mystery from the authors of the "excellent" Mr. and Mrs. North series (The New 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 . . .   A cocktail party at which no one is particularly fond of one another can be quite a bore. But when two partygoers decide to liven things up by skinny-dipping in the Townsends' pool, they stumble across a body and dive into a whole lot of trouble.   Paul J. Barlow was a man of means, and now that he's been shot in the back his wealth will go to his relatives--a fact that doesn't bode well for the couple who found the body: Barlow's niece and her husband, Winifred and Orson Otis.   The state police are summoned to the Townsend home, and Captain Heimrich intends to sort out the case right quick. But when the Otises decide to become amateur sleuths to clear their own names, they're soon in over their heads--and it's Heimrich to the rescue.   A Client Is Canceled is the 4th book in the Captain Heimrich Mysteries, but you may enjoy reading the series in any order.