In his 59th book, octogenarian Auchincloss proves he hasn’t yet burned out on one of his favorite topics: the slippery morals of upper-class Manhattanites. Set in the 1950s, ”Letters” cheerfully skewers two generations of lawyers and their wives. They couple, uncouple, cheat, and remarry, beginning with upstanding Rodman Jessup’s flagrant public affair with a socialite and his subsequent resignation (his wife is his boss’ daughter). The cynically funny story is far from a straight retelling of Hawthorne’s — and it lacks the stunning final act that made its namesake great. But Auchincloss mimics the classic in one key way: keenly observing how social status and morality interact.