Georgia O'Keeffe (1887-1986) was one of the most successful American artists of the 20th-century: her arresting paintings of enormous, intimately rendered flowers, desert landscapes and stark white cow skulls are seminal works of modern art. But behind O'Keeffe's bold work and celebrity was a woman misunderstood by even her most ardent admirers. 23 years her senior. The relationship was physically and intellectually passionate, but Atieglitz was a...