The first volume of Lomask's biography of Aaron Burr, covering his life through the disastrous Hamilton duel, was merely uninspired; the second volume, which focuses on the so-called Burr Conspiracy, founders altogether--notwithstanding the incorporation of much-publicized new evidence. What Burr intended, after his political eclipse--whether to liberate Spanish-held Mexico, to set himself up as its ruler, or to establish an independent realm in the Southwest--has never been known; the consensus today, however, is that his plans weren't treasonous. So Mary Jo Kline's recent finding that the compromising ""cipher letter"" was written by an ill-advised Burr aide, not by Burr himself, is only one more piece of evidence along that line. Lomask, in Burr's defense, makes a melodrama of the situation: ""Where was the conspiracy--in Burr's mind, or in the minds of the press and public of his day? Are we dealing with a villain, or with an early version of McCarthyism?"" (Did Jefferson himself possibly propose something to Burr--and then recant?) The mysterious story itself is well-known: Burr and General James Wilkinson, a covert Spanish agent, planned an expedition into the Southwest; with money from Anglo-Irish maecenas Harman Blennerhassett, he purchased the Bastrop grant as a base of operations. But he had talked widely about his plans, and he was under suspicion; in self-protection, Wilkinson denounced him; his party was intercepted; and he himself was apprehended and tried for treason (but not convicted). Lomask gives a plodding account of these events, and then does describe in exceptional detail Burr's long, ignominious exile in Europe (and brief, ignominious marriage to Mme. Jumel). As a research report, serviceable enough; as history or biography, feeble.