The H. H. Wiseman diary consists of sixty entries running to perhaps 3000 words, from 23 August 1864 to 12 February 1865, that describe Wiseman's experiences in several Federal prison camps as he struggles with the tedium of captivity and ever worsening living conditions. The entries touch on many aspects of prison life, including the author's longing for home, his access to information, escape attempts, relationships with guards, subjection of prisoners to northern propaganda, general living conditions, and the quality of food.

The first two entries, dated 23 August and 18 September, are retrospective; Wiseman describes his capture at Mobile and his trips to New Orleans and New York. On 9 November he begins writing on an almost daily basis. Entries 3 to 25, dated 9 November to 3 December 1864, describe Wiseman's time on Governors Island. Wiseman mentions the accessibility of newspapers and his consequent ability to follow events in the war and also the price of gold (a concern for southerners due to rampant inflation in the Confederacy). He describes the trading of personal items between prisoners and guards, being given a "very nice dinner" for Thanksgiving, and mentions that he is generally in good health.

Entries 26 to 58, dated 4 December 1864 to 5 January 1865, detail Wiseman's relocation to Elmira and the poor conditions at the prison, where nearly a quarter of the 12,100 prisoners died during the last 12 months of the war. Wiseman describes overcrowding and the barracks' inadequate heating, abusive behavior by guards (such as forcing prisoners to stand at muster in frigid weather), constant deaths among prisoners and his own deteriorating health. Many entries mention rumors of exhange and parole. The diary lacks entries from 5 January to 9 February 1865, probably reflecting Wiseman's poor health. The last entries, dated 10 and 12 February 1865, were written from the hospital at Elmira, which Wiseman describes favorably. He denies taking an oath of loyalty to facilitate his parole (which he received on the ninth). The diary appears to have been used by a Union soldier following Wiseman's death, with notations regarding letters written in April and May 1865. The new owner also wrote down the names of several individuals on the back pastedown of the diary. Research by William F. Howard, the previous owner, suggests that one of these individuals, Sgt. C. D. Allaire of the 162nd New York Infantry, was stationed at U. S. General Hospital, West's Buildings around the time of Wiseman's death and may have been the new owner.