It should be no surprise that Sontag's (The Way We Live Now) excursion into the realm of historical/romance novels serves a more rigorous agenda than merely fictionalizing the lives of Sir William Hamilton; his wife, Emma; and her lover, Lord Nelson. The narrative illuminates larger themes: the venality and hypocrisy of many of the pillars of 18th-century society; the perennial status of women as an underclass; the subservience of ethics to political expediency; the greed that often fuels a patron of the arts. These and other issues are examined in cool, ironic prose that does not disguise the author's indignation. Sontag's unconventional look at one of history's most famous amorous triangles offers revisionist portraits of her three protagonists. Hamilton, known as the Cavaliere in his post as British envoy to the Kingdom of the Two Sicilies, subverts his emotions into an obsessive urge to collect antiquities--until he becomes infatuated with Emma. Nelson is guilty of callously cruel and unprofessional behavior as a result of his infatuation with Lady Hamilton. Only she acquits herself relatively well; though she is vulgar and ostentatious, Emma has humanitarian instincts the others lack. The novel is a brilliant portrait of an age, the bloody epoch in which the Bourbon monarchs of the Kingdom of Naples--aided by the infamous Baron Scarpia of Tosca fame--took violent revenge on the revolutionaries and intellectuals who supported the insurrection of 1799. A master of descriptive detail, Sontag creates vivid pictures of an erupting Vesuvius; deadly storms at sea; the excesses of a pillaging, murderous mob. She also interjects herself into the narrative, a piquant but sometimes jarring technique. The ending, in which various characters summarize the novel's events, seems gratuitous, but it allows Sontag to drive her message home. The last line reads: ``They thought they were civilized. They were despicable. Damn them all.'' (Aug.) .