In the twentieth century, African Americans not only helped make popular music the soundtrack of the American experience, they advanced American music as one of the preeminent shapers of the world's popular culture. Vast numbers of black American musicians deserve credit for this remarkable turn of events, but a few stand out as true giants. David Stricklin's superb new biography explores the life of one of them, Louis Armstrong. Mr. Stricklin concentrates on Armstrong's musical talent, something many observers called a thing of genius. But he also pays special attention to Armstrong's identit