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." Nevertheless, they were compiled before the publication of other literature on the subject that, in several cases, caused some veterans to alter their memory and perception of events later in life. 

Officially titled, The War of the Rebellion: A Compilation of the Official Records of the Union and Confederate Armies, the Official Records are compiled in 127 volumes, plus a General Index and accompanying Atlas. Editor Robert N. Scott divided the OR into four major topical areas:

Series I contains 53 volumes (Books 1-111) and focuses on military operations. These include the battle reports for both Union and Confederate armies, arranged chronologically by campaign and theater of operations. Union reports are followed by Confederate accounts. The intent is to provide a complete history of the event in the same volume.
Series II contains 8 volumes and 8 books, and includes Union and Confederate correspondence, orders, reports, and returns relating to Prisoners of War, as well as political prisoners.
Series III contains 5 volumes and 5 books, and includes "miscellaneous" Union correspondence, orders, and reports pertaining to the organization and logistics of the Union war effort. Series III also includes calls for troops, correspondence between National and State authorities, and correspondence between Union and Confederate authorities.
Series IV contains 3 volumes and 3 books, and includes "miscellaneous" correspondence, orders, and reports of the Confederacy. Also found in Series IV are the General and Special Orders of the Confederate States Army, as well as correspondence relating to conscription and blockage running.
A word of caution must be made here about the value and limitations of the Official Records. As primary source material, the Official Records are, without question, the most complete and impartial documentation on the American Civil War. They provide a foundation for serious research into virtually any aspect of the war. On the other hand, no study of the American Civil War should rely exclusively on the Official Records. The accounts contained in the OR were not edited for accuracy, and due to space considerations, only excerpts of reports were often included. Researchers should thus verify the information found in these reports with other source material to gain as complete a picture of events as possible.

Ser. I. v. 1-53 [serial no. 1-111] Formal reports, both Union and Confederate, of the first seizures of United States property in the southern states, and of all military operations in the field, with the correspondence, order and returns relating specially thereto. 1880-1898. 111 v.--ser. II. v. 1-8 [serial no. 114-121] Correspondence, orders, reports and returns, Union and Confederate, relating to prisoners of war and to state or political prisoners. 1894 [i.e. 1898]-1899. 8 v.--ser. III. v. 1-5 [serial no. 122-126] Correspondence, orders, reports and returns of the Union authorities (embracing their correspondence with the Confederate officials) note relating specially to the subjects of the first and second series. It embraces the reports of the Secretary of War, of the general-in-chief and of the chiefs of the several staff corps and departments ... 1899-1900. 5 v.--ser. IV. v. 1-3 [serial no. 127-129] Correspondence, orders, reports and returns of the Confederate authorities, similar to that indicated for the Union officials, as of the third series, but including the correspondence between the Union and Confederate authorities given in that series. 1900. 3 v.--[serial no. 130] General index and additions and corrections.
 

[serial no. 130] General index and additions and corrections. Mr. John S. Moodey, indexer. Preface [by Elihu Root, Secretary of War]. Explanations. Synopsis of the contents of volumes. Special index for the principal armies, army corps, military divisions and departments. General index. Additions and corrections [arranged consecutively by volumes]. 1901.
NOTE: We are missing 36 books within the set of 130 books; they are book serial numbers: 1 [Charleston], 6 [Fort Pulaski-New Orleans], 12-14 [Peninsular Campaign], 21 [Chancellorsville], 39 [Port Hudson], 41-45 [Gettysburg Campaign], 60 [New Berne], 61-64 [Red River Campaign], 67-69 [Wilderness-Cold Harbor], 72-73 [The Atlanta Campaign], 75-76 [The Atlanta Campaign], 80-82 [Richmond, Petersburg, 83 [Price's Missouri Expedition], 87-89 [Richmond-Fort Fisher], 92 [Savannah], 95-97 [Appomattox Campaign], and 111 [Supplements].