"Great moments in the alphabet."  From Woroni, 1975.  See One-Letter Words: A Dictionary and even weirder: The Dictionary of One-Letter Words: The Movie.

"L is not a letter I care very much for," says Prof. Stokes in Dark Shadows.  See what you think of L's many meanings, in One-Letter Words: A Dictionary.

The Scarlet Letter is defined in One-Letter Words: A Dictionary.  From The Film Daily, 1926.

The Big U says, quoting Kina's "Get You the Moon":

Note that the Big U may have misheard the lyrics as three one-letter words: "O, U, R."

The Big U is from Wid's Daily, 1919.

From The Film Daily, 1922.

The Big U is from Wid's Daily, 1919.

Frankly, this is how we mustered up most of the W's for One-Letter Words: A Dictionary.  (Here's a photo of the genie bottle we use.)

"A, by many supposed to be the oldest of the alphabet and constituting, as it does, the initial of Adam's name, was doubtless the only letter in existence at the time Adam learned to write." —Cupid's Cyclopedia by Oliver Herford, 1910. 

The macrocosm and the microcosm?  From Tulane's 1973 yearbook.

"A is the easiest word to spell, with the exception of I."

—Cupid's Cyclopedia by Oliver Herford, 1910.  

See One-Letter Words: A Dictionary and the even more unlikely The Dictionary of One-Letter Words: The Movie.

Lady Moon's lantern, from Star People by Katharine Fay Dewey, 1910. 

The Silent E Fairy, from The See and Say Series, Book Two by Sarah Louise Arnold, 1914.

An ornate capital T, with skull.  From the Phamacy Department's page of Purdue's 1898 yearbook.  It would seem the pharmaceutical industry has never really been about your well-being.

See How to Hoodoo Hack a Yearbook.

"Z is smooth, clever, provocative[;] it is, in essence, very, very real."  From The Rotunda newspaper of Longwood College, 1970.

For many more things that Z is, see One-Letter Words: A Dictionary.

Even a skull can be a "t-bone."  From Purdue's 1898 yearbook.