How come the middle names are listed for every U.S. President except Harry S. Truman? What does the "S" stand for?
You have hit on an interesting bit of presidential trivia. Truman was given a middle initial, but no middle name. The S in Harry S. Truman refers to the names of both of his grandfathers, Anderson Shipp Truman and Solomon Young.
There has been controversy since 1962, when Truman told reporters that they should not use a period after the S, because it didn't stand for anything. He may have been joking; he himself often wrote his middle initial followed by a period. At any rate, the ruling of most style manuals is to include the period.
You can read more about this online at the Truman Library.