This site rules, because it is filled with letters written by famous, often cantankerous people the quality of which we will never see again since contemporary culture is retarding everything good.
Favorites: Apple code names a 1993 Mac "Carl Sagan," then renames it "BHA," or "Butthead Astronomer" after he bitches about his name being associated with something commercial.
F. Scott Fitzgerald admonishes his daughter not to call him "Pappy" unless she wants him to beat her cat in her absence, and for him to rename her Egg Fitzgerald. It's a very sweet letter. No, really. See here for Hark A Vagrant's Fitzgerald comic.
David O. Selznick had to write a groveling letter for permission to Rhett to speak his famous line at the end of Gone with the Wind. The word "damn" was specifically banned by the Hays Code. What arrogant, nervy censor would think that he could make them change the most explosive line in the movie after the entire world had read and fallen in love with the book and cried their ass off after reading THAT SPECIFIC LINE? What was he supposed to say, that he didn't give a darn, or a fig? OUTRAGEOUS.
Robert Burns attacks an unkind reviewer with a repetitious set of insults worth remembering for later. "Thou murderous accoucheur of infant learning"! It reminds me of this Fry rant which I like to think was inspired by that letter.
The Rolling Stones must have large beds.
There are a billion others; the site will take away hours.