There are many good sources of information about Ulysses S. Grant, most of them cataloged at the Ulysses S. Grant Association’s bibliography.
The most useful source for posts at this blog and Twitter feed is The Papers of Ulysses S. Grant, in digital form from the Ulysses S Grant Presidential Collection at Mississippi State University. However, when possible, I will also cite the primary source as given there.
One of the main primary sources is The War of the Rebellion: A Compilation of the Official Records of the Union and Confederate Armies. Following the convention of The Papers of Ulysses S. Grant, this will be abbreviated as O.R.
Other sources include:
The Ulysses S. Grant Chronology, from the same site.
Grant: Memoirs and Selected Letters, published by The Library of America.
Grant by Jean Edward Smith.
The Battle of Belmont by Nathaniel Cheairs Hughes Jr.
Where the South Lost the War by Kendall D. Gott
Ninety-Eight Days by Warren E. Grabau
Campaigning with Grant by General Horace Porter.
The Shipwreck of Their Hopes: The Battles for Chattanooga by Peter Cozzens.
Memoirs of Gen. William T. Sherman.