Reddit mentions of Ulysses S. Grant : Memoirs and Selected Letters : Personal Memoirs of U.S. Grant / Selected Letters, 1839-1865 (Library of America)

I can second /u/caffarelli's recommendation of Seven Pillars.

I really enjoyed reading Ulysses S. Grant's Memoirs. I have the Library of America version, but there are many of them.

As for letters, you might be interested in the blog Letters of Note which is all about interesting and imporrtant letters from history.

I've heard his memoirs are excellent: http://www.amazon.com/Ulysses-S-Grant-Selected-1839-1865/dp/0940450585

The note on the gift Santa gives you says:

"To Irvette,

You wanted books, so here are some items."

Inside the present is The Mouse Rap with a note that says "Just read the slang....", The autobiography of Ulysses Grant, and Moby Dick, cause yeah. ..

James McPherson’s Battle Cry of Freedom is immediately the first book that comes to mind. As mentioned elsewhere in this thread, it is “the book” to read on the Civil War. It is a highly readable account of the build-up to the Civil War, causes, and the war itself. It also won a Pulitzer Prize. For more, I’d also check out Ta-Nehisi Coate’s online book club on Battle Cry of Freedom over at The Atlantic.

Other excellent works on the period I would recommend are:

• Team of Rivals by Doris Kearns Goodwin: an account of the Lincoln administration during the war years
  
  
• The Fiery Trial: Abraham Lincoln and American Slavery by Eric Foner: details Lincoln’s career and his relationship and views on slavery.
  
  
• Fall of the House of Dixie by Bruce Levine: takes a look at the southern plantation economy and its destruction in the Civil War
  
  
• This Republic of Suffering by Drew Gilpin Faust: Harvard President and historian Faust looks at how the nation collectively dealt with the death of 600,000 young men and the national trauma of the war
  
  
• Lincoln and His Generals by T. Harry Williams: an older book, but still a classic on the Union command structure and Lincoln’s difficulty in choosing an effective commander for the Union Army
  
  
• Shelby Foote’s Civil War trilogy: for the military side of the conflict without much historiography
  
  Also, the Civil War produced some of the greatest memoirs in American letters:
  
  
• Grant’s Memoirs: written after his presidency with the assistance of Mark Twain, who later compared them to Caesar’s Commentaries
  
  
• Sherman’s Memoirs: called by literary critic Edmund Wilson a fascinating and disturbing account of an "appetite for warfare" that "grows as it feeds on the South"
  
  
• The Rise and Fall of the Confederate Government by Jefferson Davis: a massive tome of a book in which Davis lays out his rational for secession (in hindsight) and upon which much of the Lost Cause mythology would later be based
  
  And, I always recommend reading poetry and fiction, so I would also encourage you to look at Stephen Crane’s The Red Badge of Courage, as well as the war poetry of Walt Whitman and Herman Melville, particularly Melville’s poem The Martyr, written days after Lincoln’s assassination. More contemporary fiction would be Michael Shaara’s The Killer Angels, or EL Doctorow’s The March.
  
  Finally, check out David Blight’s Open Yale Lectures on the Civil War. Prof. Blight is a fantastic lecturer. They are free, and the course syllabus is online, and in 26 hours you can take a full Yale course completely on your own.