The War of the Rebellion: Original Records of the Civil War
As the site says:
No serious study of the American Civil War is complete without consulting the Official Records. Affectionately known as the "OR", the 128 volumes of the Official Records provide the most comprehensive, authoritative, and voluminous reference on Civil War operations. The reports contained in the Official Records are those of the principal leaders who fought the battles and then wrote their assessments days, weeks, and sometimes months later. The Official Records are thus the eyewitness accounts of the veterans themselves. As such they are "often flawed sources – poorly written in some cases, lacking perspective in others, frequently contradictory and occasionally even self-serving."