An aggregate of princely portraits limning petty rulers of Italian states whose unpleasant-visages crop up from time to time in cultural monuments of the Renaissance. With the exception of the Borgias and the Medicis, who have accrued enormous biographical attention, some of the others, despite trumpeting titles (Bodovico Maria Sforza, Duke of Milan, Isabella d'Esc Marchioness of Mantua, Alfonzo of Naples, etc.), have had relatively short shrift except as walk-ons in historical novels. Short shrift is what they're getting again. Each prince's nefarious to pietistic activities (the pietistic were rare) in love, war and political maneuvering are given in outline, with a contemporary letter or comment or two, a physical portrait, and one or two usually horrific blood-lettings. Spurred on by the author, a composite is soon reached -- the Prince is hawk-nosed, sentimental, brutal, greedy, often very shrewd and fond of large processions. Obviously a labor of love for the retired book critic of the New York Times, whose humor and discernment relieve the clumps of research. But one wishes he had allowed himself a freer rein in fleshing out the personalities.