Captain Heimrich resolves to get to the bottom of a New Year's drowning 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 New Year's Eve party should be filled with champagne, laughter, and the good company of others. And the Halley's celebration at their country house is exactly that. But the next day has barely dawned when a shocking discovery begins the new year with a dreadful bang.   Psychologist Margaret Halley is convinced her depressed husband drowned himself in the lake. But why would a man intent on suicide walk through a hundred yards of deep snow to jump into a freezing lake when there was a bottle of sleeping pills next to his bed?   As Captain Heimrich investigates the Halleys--and their guests--he has a sinking feeling that someone must have sent John Halley to his watery grave.   Let Dead Enough Alone is the 9th book in the Captain Heimrich Mysteries, but you may enjoy reading the series in any order.