I got to meet Tony Hillerman at Bouchercon a few years ago. Even then he moved slowly and seemed a bit shaky. But his mind was as sharp as ever.
When I got my first contract in 1997, I panicked. I had written a book, which the publisher had decided was a mystery, and now they wanted another. How was I going to do that?
I read all these books on mystery writing. One that particularly struck with me was called Writing Mysteries, and I photocopied the chapter from Tony Hillerman called Building Without Blueprints. Every time I read it, I find another piece of advice I can use. You should be able to read it here.