From the memoir:

On the 30th of September I reported for duty at Jefferson Barracks, St. Louis, with the 4th United States infantry. It was the largest military post in the country at that time, being garrisoned by sixteen companies of infantry, eight of the 3d regiment, the remainder of the 4th. Colonel Steven Kearney, one of the ablest officers of the day, commanded the post, and under him discipline was kept at a high standard, but without vexatious rules or regulations. Every drill and roll-call had to be attended, but in the intervals officers were permitted to enjoy themselves, leaving the garrison, and going where they pleased, without making written application to state where they were going for how long, etc., so that they were back for their next duty. 

It did seem to me, in my early army days, that too many of the older officers, when they came to command posts, made it a study to think what orders they could publish to annoy their subordinates and render them uncomfortable. I noticed, however, a few years later, when the Mexican war broke out, that most of this class of officers discovered they were possessed of disabilities which entirely incapacitated them for active field service. They had the moral courage to proclaim it, too. They were right; but they did not always give their disease the right name.

Also, I received a note from Aaron Lisec who edited Grant's papers for 15 years. I asked Lisec if there was any truth to the charge that Twain "wrote" the memoir:

Completely baseless. Aside from the consistency with his diction and syntax, much of the manuscript survives, in Grant's hand, and what isn't in his hand is accounted for by letters describing secretaries hired to take dictation. You may know that the Memoirs originated in four articles Grant agreed to write for the Century Magazine, which ran a series on civil war battles told by the generals involved. He had turned down many such offers before, citing laziness, and only agreed after he lost all his money in May 1884, and had to raise funds fast. 

In Volume 31 of the Grant Papers, we document how he turned in the first article, on Shiloh, and had to be gently told that it sounded like an official report--the editors came down to his summer house in Long Branch and coaxed him into rewriting it in his own voice, with his own observations. You can see how the article changed after that, and how much better the next one was, and about that time he was diagnosed with cancer and decided to turn the whole thing into a memoir, forgoing the last two articles. I could go on, but you get the gist.