Reference resources recommended by Albert Hallenberg of the History and Genealogy Department of the Public Library of Cincinnati and Hamilton County:
Who Was Who in the Civil War by Stewart Sifakis
Biographical Dictionary of the Union: Northern Leaders of the Civil War by John T. Hubbell and James W. Geary
Biographical Dictionary of the Confederacy by Jon L. Wakelyn
Civil War Dictionary by Mark Mayo Boatner
Historical Times Illustrated Encyclopedia of the Civil War by Patricia L. Faust
Civil War Day by Day: An Almanac, 1861-1865 by E.B. Long with Barbara Long
Civil War Almanac by John S. Bowman
Chronological Tracking of the American Civil War per the Official Records of the War of the Rebellion by Ronald A. Mosocco
A Compendium of the War of the Rebellion by Frederick H. Dyer
Compendium of the Confederate Armies by Stewart Sifakis
Union Army, 1861-1865: Organization and Operations by Frank J. Welcher
War of the Rebellion: A Compilation of the Official Records of the Union and Confederate Armies (128 volumes, plus 95-volume supplement and five-volume index)
Official Records of the Union and Confederate Navies in the War of the Rebellion (30 volumes)
Roll of Honor: Names of Soldiers Who Died in Defense of the American Union, Interred in the National Cemeteries from the US Quartermaster's Department
Roster of Union Soldiers, 1861-1865 by Janet B. Hewett
Roster of Confederate Soldiers, 1861-1865 by Janet B. Hewett
The Sable Arm: Negro Troops in the Union Army by Dudley Taylor Cornish
Woman's Work in the Civil War: A Record of Heroism, Patriotism and Patience by L.P. Brockett and Mary C. Vaughan
The West Point Atlas of the Civil War from the US Military Academy
The Atlas of the Civil War edited by James M. McPherson
Atlas to Accompany the Official Records of the Union and Confederate Armies from the US War Department
Photographic History of the Civil War by Francis Trevelyan Miller
The Civil War Times Illustrated Photographic History of the Civil War edited by William C. Davis