The Speed of Dark, by Elizabeth Moon.
Lou Arrendale is autistic. He was born too early to get the treatments that cured autism in infancy but not too late to get the therapy that made him high-functioning. He lives a fairly normal life, works for a pharmaceutical company finding patterns in data, goes to fencing practice once a week. But now the company wants him, and the other autists in his section, to get an experimental treatment that will make him normal. But, if he becomes normal, will he still be Lou Arrendale?
While I was reading it, I didn’t know how it was going to end. As I neared the end, I didn’t know how I wanted it to end. And when I got to the end, I cried. And I still don’t know why.
Nebula Award 2003. Highly recommended.