This is a wonderful online historical collection from Mississippi State University and the Ulysses S. Grant Association including The Papers of Ulysses S. Grant. This deep archive allows you to read all 31 volumes of his papers made up of thousands of letters and documents. The collection also includes photographs, sheet music and political cartoons.
The digitized documents are accompanied by information about the 18th President including a timeline, his family genealogy, military service and a bibliography. Truly an interesting and detailed source on the life and times of Ulysses S. Grant.