After Abraham Lincoln's assassination in 1865, William H. Herndon began work on a brief, subjective biography of his former law partner, but his research turned up such unexpected and often startling information that it became a lifelong obsession. The biography finally published in 1889, Herndon's Lincoln, was a collaboration with Jesse W. Weik in which Herndon provided the materials and Weik did almost all the writing. For this reason, and because so much of what Herndon had to say about Lincoln was not included in the biography, David Donald has observed, To understand Herndon's own rather...