The Diary of Horatio Nelson Taft is housed in the Manuscript Division at the Library of Congress. Mrs. Willoughby Davis, a Taft descendant, donated the diary's three volumes as a gift to the Library in 2000.

The three volumes comprise about 1,240 digital images in the online collection and span the years 1861 to 1865. Taft wrote daily from January 1, 1861, through April 11, 1862, and irregular entries thereafter until May 30, 1865.

The diary documents Taft's life in Washington, D. C., where he worked as an examiner in the U.S. Patent Office. It is especially significant because of his connection to the Abraham Lincoln family and because of his descriptions of daily life in Washington during the Civil War. Its contents provide details about Taft's family life and various events in Washington, including descriptions of the arrival and quartering of regiments, hospitals, and the daily news reports (sometimes inaccurate) of battles. Included in the diary is a report of President Lincoln's assassination based on accounts Taft received from friends and particularly his son, Charles Sabin Taft, a U.S. Army surgeon who was in Ford's Theatre the night Lincoln was shot. He was one of the physicians in attendance on Lincoln throughout the night of April 14, 1865.

Transcriptions for all three volumes of the diary have been made by Library of Congress staff and are available online along with the digital images. Horatio Nelson Taft's written narration presented a challenge to transcribers. Those who wish to compare the manuscript diary with the transcription should note that many "stray" marks have been ignored and treated as "pen rests." Taft's remarks often form a series of unpunctuated phrases that require some type of clarification. A limited amount of punctuation and capitalization has been introduced in the transcription, chiefly to clarify text or begin new sentences. Inconsistent capitalization within sentences and misspellings have been left largely as they appear in the hand of the diarist. Missing quotations have been supplied and confusing abbreviations expanded. Many personal names in the diary appear with variant spellings, and nicknames for family members are often used. These will be identified in the published version of the diary sponsored by the Lincoln Studies Center at Knox College in Galesburg, Illinois.