In the autumn of 1835 Americans looked up and marveled as Halley's Comet—not due to reappear for another seventy-five years—illuminated the skies. On 30 November, as the comet passed overhead, a small-town lawyer, merchant, and real-estate developer named John Marshall Clemens welcomed a baby son into the world. John Clemens and his wife, Jane Lampton Clemens, had moved from place to place during the first twelve years of their marriage, settling in 1835 in the sleepy hamlet of Florida, Missouri. Young Samuel Langhorne Clemens joined a household that already included two older brothers, two older sisters, and a slave named Jenny. In 1839 the family moved thirty miles east to Hannibal, a bustling town of a thousand-odd residents. In this town perched on the banks of the Mississippi River, young Sam Clemens spent his childhood, absorbing the speech and spirit of antebellum life on...