Due to give a lecture series at Harvard, Oxford don Ambrose Usher (sometime Intelligence agent) is asked to investigate old Boston acquaintance George Fletcher--a tycoon all set to take over some British companies with heavy corporate connections in Eastern Europe. Fletcher, it soon appears, is tied to the Irish Mafia, and thus to godfather John Harrigan--a recluse with a mania for acquiring fine-art representations of St. John the Baptist. Harrigan's latest prize: a Donatello sculpture stolen from a Sienese cathedral. So Usher--in between lectures, lunches, dinners, and assignations with charming art-historian Alyss Summers--manages a bit of inept spying. But eventually it's Treasury head McConnochie (a tiny dandy and compulsive gambler), with assists from Alyss' ex-CIA nephew Tim, who puts Fletcher on the run and retrieves the Donatello. The first Usher outing in some years (and the first to appear here in decades), this is even more heavily weighted with voluble erudition than the early books: multi-lingual allusions, disquisitions on economics and philosophy (Usher was once rumored to have been modeled on Isaiah Berlin), esthetic chat of all sorts. Slow going, then--less for mystery readers than for those eager to O.D., fairly painlessly, on Kultur.