I just finished Robert Spaethling's terrific translation of Mozart's letters. He really was one of the great letter writers, up there with Keats and Byron. Here are a few things I learned:
1) He was clever. He casually apologizes to his father for sending him a piece with the prelude and fugue in the wrong order: 'The reason for the mixup is this: I had already composed the fugue and was writing it down while I worked out the prelude in my head.' That sounds hard.
2) He was confident: for a performance in front of the Emperor, he wrote a violin sonata so quickly that he had to improvise the piano part himself.
3) He was fast: on a visit to Linz he was suddenly asked to give a concert of one of his symphonies. But he hadn't brought his music with him, so he wrote one of his greatest symphonies, no. 36 (the 'Linz'), in three days.
4) He didn't much care for nature. In March 1778, he and his mother took the coach from Mannheim to Paris. It took nine and a half days: 'I've never been so bored in all my life.' According to Googlemaps, you could now drive it in less than five hours. And you could listen to Mozart on the way.
5) In some of his letters he uses scatological language that would make Norman Mailer blush. But, oddly, he never uses it to male correspondents, or to his wife. It was just to his female cousin - and to his mother.
6) And putting my crime writer's hat on for a moment: he wasn't killed by Salieri.