I just found this fantastic website that "attempt(s) to gather and sort fascinating letters, postcards, telegrams, faxes, and memos" of famous people from Conan O'Brian to Adolf Hitler. Seriously, a fantastic collection of primary sources.
Letters of Note
For instance, there is one letter Albert Einstein wrote to the philosopher Erik Gutkind where, after reading the latter's book, he wrote (among other interesting criticisms and points), that;
Albert Einstein to Erik Gutkind wrote:
Here is another one I found really amusing. I honestly don't have any idea as to the author's book or its content, but something tells me I would really get along well with him. The sign-off with the blurb in question is classic.
Harlan Ellison to Amy Stout wrote:
12 February 92
You may use this on Connie's new book. In full, no ellipses, no altering for any edition. And send me a copy of the book, please. And warn the troll drones in your art and publicity departments that if they f*ck around with the quote, change it or alter it for a meaning not intended, I will come and find them and kill them so dead I'll murder their ancestors!
"The writing of Connie Willis is irresistable. Clever, cunning, and capable of tantalizing the most demanding reader."
Letters of Note: Don't F*ck With The Quote
Give the site a look through and post anything interesting you find. I found some other really good letters from George Washington and so on, so you are sure to find something interesting.