Blackwell Dictionary of Historians.  D 14 .B58 1988
Contains over 450 biographical entries that indicate the scholarly reputation of historians, the circumstances in which they worked, and the extent to which their work has subsequently been confirmed or refuted.

Dictionary of American Biography.  E 176 .D56
This 30+ volume set is the most authoritative of the major biographical dictionaries of American history, and provides biographies of over 13,600 Americans, none of whom are living today. Use this in addition to the American National Biography Online because different points of view are represented in each set.

Notable American Women: A Biographical Dictionary.  CT 3260 .N57
A 3-volume set that provides biographical articles on over 1,800 deceased women from the United States, many of whom are not represented in the standard biographical source, The Dictionary of American Biography. A fourth volume covering the modern period was added in 1980. Signed scholarly articles with bibliographies.

Frederick Jackson Turner (1861-1932) was a prominent American historian in the early 20th century.  He is best known for his work The Significance of the Frontier in American History, more commonly known as the "Frontier Thesis." He originally presented his ideas in a paper at the American Historical Association meeting in Chicago in 1893, held during the World's Columbian Exposition.

(Attributed to Magnus Manske; Frederick Jackson Turner, Wikimedia Commons)