John Nicolay was Lincoln's secretary and biographer. A short bio can be found at Wikipedia. Nicolay had met Mr. Lincoln when he worked at the free press in Pittsfield, Illinois.
You can find more information about Mr. Nicolay at http://www.mrlincolnswhitehouse.org/inside.asp?ID=21&subjectID=2
Several Books written by Nicolay can be found at Project Gutenberg.
See the bibliographies on these websites, and then check OCLC Worldcat, NetLibrary, and Ebrary for the library's copy. Otherwise, submit an ILL book request form! (See Books and eBooks tab!)
John Hay, the Assistant Private Secretary to Abraham Lincoln, co-authored the 10-volume Abraham Lincoln: A History. He was clerking his uncle's law office in Springfield in 1859-60 when he came to know President-elect Lincoln. John G. Nicolay, Mr. Lincoln's secretary, insisted that Hay accompany them to Washington. Mr. Lincoln acquiesced in hiring the youthful graduate of Brown University. During Lincoln's presidency, Hay was a social companion of Robert Lincoln when the President's son was in the capital. In 1863 and 1864, Hay served on military missions to South Carolina and Florida and was appointed an army major to investigate an insurrection plot in St. Louis. (Lincoln and Friends)
Short Bio on Wikipedia
Many of the items John Hay collected when he was Lincoln's Secretary at housed at the John Hay Library, Brown University. They include pamphlets, notes and letters written by Lincoln, and other items.
Assistant secretary to President Lincoln (1864-65), Edward Duffield Neill was an ordained Presbyterian minister and dedicated historian. At the beginning of the Civil War, Rev. Neill served as chaplain to a Minnesota regiment and later as an army hospital chaplain in Philadelphia. (Lincoln and Friends)
The Family Papers for Edward Neill are at the Minnesota Historical Society. An inventory list can be found at http://www.mnhs.org/library/findaids/p2668.xml
He also wrote a book. Abraham Lincoln and his mailbag / two documents by Edward D. Neill, One of Lincoln's secretaries ; Edited by Theodore C. Blegen