After more than a decade's absence, Dann (The Man Who Melted, 1984, etc.) returns with a big, ambitious historical fantasy based on the career of Leonardo da Vinci. A frame story begins with Leonardo on his deathbed, searching over the memories he's stored in a large imaginary cathedral--a method of enhancing memory practiced by many Renaissance thinkers. His entrance into the cathedral takes him back, in memory, to the years of his apprenticeship in Florence. There, we find the 25-year-old Leonardo establishing a reputation as artist and engineer and falling in love with a young woman betrothed to a wealthy older man. He's also working on plans for a flying machine, and the Medicis, who are planning war against the papal states, have expressed an interest in its military potential. Just as he appears to have his foot on the ladder to success, it is yanked out from under him: An anonymous accusation subjects him to a trial for sodomy and kills his hopes for advancement. At this point, Dann takes advantage of a gap in the known details of Leonardo's life, and sends him to the Orient--as military advisor to the Caliph of Syria, employing his inventions (including the flying machine, which finally works) in war against the Turks. It's here that the story reaches its most fantastic heights, with a profusion of exotic detail (the chapters on Italy are no less detailed, but the background is perhaps more familiar). At the same time, we see Leonardo facing the consequences of the inventions that, in real life, he saw only as sketches that his patrons ignored. Historical facts and conjectures are seamlessly woven together, and the cast of characters is a Renaissance Who's Who. The author's return reveals a new maturity and imaginative scope only hinted at in his earlier work. An impressive accomplishment.