Paulsen says that he wanted to write a biography of Sally Hemings, a slave who may have been Thomas Jefferson's mistress. He could not find enough information on her to write a biography, but during his research, he found a book of interviews with former slaves. This book inspired Nightjohn (please see separate entry), which in its turn inspired Sarney (please see separate entry). Research for these books inspired his interest in the Civil War, and his further research into the war led him to the historical figure Charley Goddard, who at age fifteen enlisted in the Union Army. The outlook and tone of Soldier's Heart is foreshadowed in Paulsen's The Rifle (please see separate entry), a history of a beautiful and deadly weapon.