The rejections that I got were increasingly better rejections, which is not to say good, but the editors who saw things asked to see the next manuscript. So there was always a little bit of encouragement. For most jobs, you go to school or there's an apprenticeship period or something like that. For a mystery writer, you're really on your own. It's very much independent study and trial and error.
Some people have writing backgrounds.
I didn't, other than being an English major. So I really learned as I wrote, and I could see afterwards some mistakes that I'd made in one book that I wasn't going to make again. I could see the possibilities that I hadn't tried. I did change characters. That was one of the things that I saw - the first three books, the character was a social worker, which is what I was.