Self-Portriat of the assassin. On a winter night in 1537, a young man cut the throat of the Duke of Florence. The mystery was never who-did-it, but why.

In this brief novel, the author provides a more elegant explanation than the one that most historians give. The Duke was no great loss. He was coarse and heavy-handed, remarkable only for his tireless sexual depredations. The murderer was his distant cousin, Lorenzino de' Medici - his companion, courtier, and sometimes procurer. Scholars say that Lorenzino killed the Duke out of jealousy.

The historical Lorenzino tried later to claim that he was a modern Brutus, liberating the city from the grip of a tyrant. But if that was what he intended, why did he run from Florence like a cirminal instead of staying to organize a genuine revolt? This novel is Lorenzino's account, written in hiding in Bologna a few days after the killing. There is not much movement or noise in this book, but rather the steady murmur of voices in recollected conversations - intelligent but rather abstract and sometimes monotonous.

Arvin Upton has written a novel of ideas, and the central idea is political power. The Duke had a great deal of it, and wantonly misused it. Lorenzino was his hanger-on, and the beneficiary of the power. Eventually the relationship turned murderous. The author states the motive clearly but - the central defect of the book's style - does not quite manage to demonstrate it persuasively.

Lorenzino must have known more about the nature of power than he tells the reader. The base of the Duke's authority was not only eminent relative in Rome, Pope Clement V11, but the Hapsburg Emperor's Spanish infantry who had conquered Florence for him. The novel's jacket subtitle says that it is about "Florence in the Renaissance." But during Lorenzino's lifetime, the Renaissance ideal of the city republic died for Florence, and the baroque practice of the absolutist state commenced. (Norton, $7.95)