I saw this first quote in McCullough’s Truman book (pointed out to me by my lovely wife) so I went looking for confirmation of it on-line and found the other quotes as well:
Always do right; this will gratify some people and astonish the rest.
– Note to the Young People’s Society, Greenpoint Presbyterian Church, 1901
Do right and you will be conspicuous.
– Mark Twain, a Biography
Do right for your own sake and be happy in knowing that your neighbor will certainly share in the benefits resulting.
– What Is Man?
See more of Twain at: http://www.twainquotes.com