The book itself focuses primarily on Grant’s military career, as you might expect. Grant was a soldier in the Mexican-American War in the late 1840s, and became the leader of the Union Army in during the Civil War in the 1860s.
While it is an interesting factoid that Mark Twain published Grant’s memoirs, it isn’t the most important thing you should know. While it is likely that the two-volume set would have been successful no matter what, given who the author was and his popularity, Mark Twain devised a marketing plan that allowed the book to become wildly successful.
His plan utilized former veterans who canvassed the Northern states, usually dressed in their old military uniforms, selling the books. This allowed them to sell both initial printings, around 350,000 books total.
At the time of his death, and really since the end of the Civil War, Ulysses S. Grant was very popular in the North (not so much in the South, for obvious reasons). With so many former soldiers (due to the size of the Union Army), almost everyone knew someone who had fought in the war. Twain used this popularity to increase book sales. In addition, each set included a letter that appeared to be a handwritten note from the great General himself, which increased the popularity.
In the end, the memoirs were very popular at the time, and continue to be so to this day. If you really want to get into the history of the Grant family after Ulysses S. Grant’s death in 1885, you will find that it is very entertaining reading. He had several descendants who took his name, the most recent of which died in 1977 (Ulysses S Grant IV). Unsurprisingly, most of them served in the military.
The memoirs provided financially for the future of the Grant family, with Mark Twain’s help, and continue to this day to impress military historians the world over.