One became America's greatest writer. The other died in obscurity and failure. As brothers, they shaped each other's lives and work. In a compelling way, Philip Fanning traces the fraternal relationship of Orion and Samuel Clemens from its beginning in Hannibal, Missouri, in the 1830s to Orion's death in Iowa in 1897. He demonstrates that Orion's influence on the writer known as Mark Twain was profound, pervasive, and prolonged.  In some respects, Samuel defined himself against Orion's formidable background. It was Orion who became the chief financial and spiritual support of the Clemens family following the father's controversial death in 1847. It was Orion who led the way for his brother into printing, journalism, and mine speculation. And it was Orion who served as Sam's first real editor and literary mentor, recognizing and encouraging his younger brother's talents as a writer.  The two siblings had much in common, and they often appeared to be codependent, so much so that their attitudes veered sharply from mutual admiration to mutual disdain and rivalry. Whereas Orion was self-effacing, easygoing, humble, and adventurous in his politics and progressive in his views, Twain was often ill-tempered and antagonistic toward those around him and conservative in his outlook. He frequently portrayed his older brother in autobiographical writings and letters as a buffoon and a laughingstock.  Fanning--who drew upon extensive archival sources, unpublished letters between the two brothers, and the Mark Twain Papers at the University of California, Berkeley--charts these divergences in their characters and in their fortunes. As Twain rose to become a national celebrity and a financial success, Orion's finances and self-esteem disintegrated, and Twain's treatment of his brother became evermore harsh and mocking. Fanning's study stands as both a biography of a fractious fraternal relationship and a work of scholarship that highlights for the first time how significantly Orion Clemens influenced Twain's psychic and artistic economy.