"Didn't he pass away fairly recently?"
I haven't heard that he did. I would imagine my horse racing clubs and web sites would have said something if he had.
I just did a quick search and his wife had a heart attack and passed away in 2000. She helped him with a lot of his research.
So, who knows if he'll want to write another book. It might be too hard - from an emotional point of view, I mean.