By WALTER ISAACSON
Exerpt: He was slow in learning how to talk. “My parents were so worried,” he later recalled, “that they consulted a doctor.” Even after he had begun using words, sometime after the age of 2, he developed a quirk that prompted the family maid to dub him “der Depperte,” the dopey one. Whenever he had something to say, he would try it out on himself, whispering it softly until it sounded good enough to pronounce aloud. “Every sentence he uttered,” his worshipful younger sister recalled, “no matter how routine, he repeated to himself softly, moving his lips.” It was all very worrying, she said. “He had such difficulty with language that those around him feared he would never learn.”