When Doris Kearns Goodwin was still young and unknown and writing her biography of former President Lyndon B. Johnson, she stayed at his Texas ranch. Sometimes, she said in the book’s prologue, when he could not sleep, he would settle into her bed and confess his troubles while she sat nearby.

Walter Isaacson was at Steve Jobs’s bedside as Mr. Jobs was dying of cancer, an experience, Mr. Isaacson has acknowledged, that made him “deeply emotionally wrapped up” with his subject.

Contemporary biography has always been a tricky balancing act, even before Paula Broadwell demonstrated with her book about David H. Petraeus how the scales can tip decisively the wrong way....