"I must meet that man," Leni Riefenstahl used to say whenever she sensed that a powerful male might advance her film career or enable her to navigate the tricky political and social minefield of pre–World War II Berlin. Seduced by her beauty and daring, one lover or besotted mentor after another helped her scramble her way up from a modest dancing career to movie stardom as a barefoot, mountain-climbing maiden in the kitschy, popular "Alpine" films. But Riefenstahl's sights were set higher than the peaks she scaled in these sentimental melodramas. Eventually, she decided to become a director—and the man she resolved that she "must meet" was Adolf Hitler.   The result, as Steven Bach tells us in his fascinating biography Leni(Knopf), was Triumph of the Will, Riefenstahl's morally repellent but aesthetically gorgeous masterpiece documenting the monumental rally that the Nazis staged in Nuremberg in 1934. As the darling and favorite filmmaker of Hitler and his henchmen, Riefenstahl made movies that included a stirring record of the 1936 Berlin Olympics and another, less well-known feature, Tiefland, for which she recruited Gypsies from a nearby detention camp to work as extras. Bach does a marvelous job of unraveling Riefenstahl's tangled love affairs and of describing the extraordinary lengths to which she'd go to get the shots she wanted. Throughout, he makes a compelling attempt to fathom this endlessly enigmatic woman who always maintained she'd known nothing about the Nazi genocide, and who, in old age—she lived to be 101—managed to reinvent herself with her underwater filming and her celebrated photos of the Nuba tribe in the Sudan. Ultimately, Leni is a cautionary tale about an artist whose prodigious determination and ambition seem to have been unmediated by the slightest influence of conscience, soul, or heart.