I don't think the fact that William Dean Howells wrote an 1860 campaign
biography _Life of Abraham Lincoln_ should be overlooked. This book is not
listed in Gribben's _Mark Twain's Library: A Reconstruction_. However in a
letter dated 5 Aug. 1876, Howells reminded Clemens: "You know I wrote the
Life of Lincoln which elected him." The text of Howell's book is also
available online at archive.org.