This book dazzles by walking a dangerous high tightrope pulled taut between the widely separated poles of the story. The fairy-tale childhood, with its startling yet archetypically resonant improbabilities, has to consort with the hacker realities of the Kurt-based story, which in itself is not overtly unlikely, but still slightly gonzo. But, like the best mashup tunes, Doctorow’s narrative wedges the most consensually disparate elements together into a brilliant whole.