Origin of scores for letters in Scrabble (in relation to their frequency in a langauge)
That is touched upon in the official History of Scrabble by Hasbro, the owner of Scrabble.


  Legend has it Butts studied the front page of “The New York Times” to make his calculations for the letter distribution in the game. This skilled, cryptographic analysis of our language formed the basis of the original tile distribution, which has remained constant through almost three generations and billions of games.


I have misplaced (or more likely given away, again) my copy of Everything Scrabble, but I remember bits of what I think I remember reading there (hows that for reliable source?).

From what I remember reading, the letter distribution in Scrabble has continuously slid in favor of people who memorize things like 2-Letter Lists and Q without U words. Even with two well-read players who have never really studied the English language as it relates to Scrabble, the distribution is off-kilter for the normal vocabulary in 2015. 

It's not your imagination, there are too many Is. People who know that QI and ZA are words, really tip the value (and risk) of those letters.

Only Hasbro can change Scrabble, but Words With Friends is gaining popularity in part because the tiles more closely represent the way the English language is used today. 

Should the tile values change with the times?

Is ZA really a word?

Has X become the best letter in the alphabet?

These are hotly debated subjects.

The history of how the tiles got their distribution seems pretty clear, even if it is described in the official history as a legend. What is less clear is the origin of the point value of individual letters. I think the point value is a part of the same thing, unchanged since the Scrabble name, but that the values were printed on the scorecard, not the tiles. Unfortunately, I have no citation for that, it's one of those things that I read somewhere - or maybe I saw an antique scorecard on display - who knows? But I am fairly certain that the official Scrabble point value per letter has remained unchanged along with the distribution of 100 letters.