The Nicolay-[John] Hay biography has had lasting historical significance. Both Hay and Nicolay were partisan Republicans, writing under the watchful eye of the martyred president's son in a period when the ideal of historical objectivity had yet to be fully established. In delivering the manuscript to Robert Todd Lincoln, Hay and Nicolay assured him that "every line has been written in the spirit of reverence and regard." Nevertheless, the biography is useful, because while the authors' conclusions are predictable, they are generally grounded in documentary sources and historical records. Additionally, they provide important eyewitness evidence of Lincoln's shifting moods and anxieties, particularly his gloom over the uncertain outcome of the 1864 election. There is no question that the Nicolay-Hay biography reflected the partisanship of the period and has a strong pro-Lincoln bias. It contains intense criticism of General George B. McClellan, the Copperheads, and the Radical Republicans, and it depicts Lincoln as a grand, almost mythical figure. Nevertheless, the biography made an important contribution to Lincoln scholarship. While far too admiring, Nicolay and Hay were the only biographers to have access to Lincoln's papers for more than fifty years. The biography also played an important role in the Civil War historiography of the late nineteenth century, shaping interpretations and prompting attacks by other biographers, such as William Herndon and Jesse W. Weik. Together Nicolay and Hay also edited Lincoln's writings, which were published in two volumes in 1894 and later enlarged to twelve volumes.
Daniel Hamilton, "Nicolay, John George," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00744.html.