One of the most amazing things I've seen lately, it's a collection of letters between Gehrig and his doctor during his two-year battle against amyotrophic lateral sclerosis, or ALS. The first couple letters, including one that you can see in PDF format, are in his handwriting. But the rest apparently are typed or dictated as he gradually lost his motor skills.
The letters, and the article on the Wall Street Journal's website (I don't know how long that link will last), really show how fragile but also how brave Gehrig was.
The article ends with probably the most poignant and wise except, taken from a letter he wrote 5 months before he died just shy of his 38th birthday:
"Don't think that I am depressed or pessimistic about my condition at present. I intend to hold on as long as possible and then if the inevitable comes, I will accept it philosophically and hope for the best. That's all we can do."