Books like Robert Heinlein's Citizen of the Galaxy

Just outside our galaxy the atrocities of slavery thrive, and young Thorby is just another orphaned boy sold at auction. But when he crosses paths with a mysterious crippled beggar, his destiny is forever changed. Citizen of the Galaxy is an adaptation of the interstellar action/adventure, coming of age tale by the "Dean of Science Fiction"... Robert A. Heinlein!