In 1600, Rome's Campo de' Fiori, now a nice plaza lined with cafés, was one of the city's execution grounds, and on Ash Wednesday of that year Giordano Bruno, a philosopher and former priest accused of heresy by the Inquisition, was taken there and burned. The event was carefully timed. Ash Wednesday is the primary day of Christian penance. As for the year, Pope Clement VIII chose it because 1600 was a jubilee for the Church—a festivity that would be enhanced by the execution of an important heretic. Bruno rode to the Campo on a mule, the traditional means of transport for people going to their death. (It was also a practical means. After years in the Inquisition's prisons, many of the condemned could not walk.) Once he arrived and mounted the pyre, a crucifix was held up to his face. According to a witness, he turned away angrily. He could not speak; he had been gagged with a leather bridle. (Or, some say, an iron spike had been driven through his tongue.) He was tied to the stake, and the pyre was lit. When it had burned out, his remains were dumped into the Tiber. As Ingrid Rowland writes in "Giordano Bruno: Philosopher/Heretic" (Farrar, Straus & Giroux; $27), the Church thereby made Bruno a martyr. But "a martyr to what?" she asks. That is the question that her book, the first full-scale biography of Bruno in English, tries, with difficulty, to answer