Books have to start with an idea.  Ideas come from all sorts of places.  What if elephants had wings?  What if a jury was electronic?  What if I won the lottery?  I have used the image of the chicken crossing the road before.  Simple: a chicken crossed the road.  A fat chicken crossed the road.  A fat chicken crossed the dirt road on a snowy day.  And so on.  You can build a whole mystery around that chicken.

This story, Death at the Edge of the Diamond, arises from a young man who is coming of age, having fun, on Cape Cod in the summer of 1979 who is confronted by a murder.  Not many of us have been confronted by murder in real life.  So right there.  That's unusual - at least in real life.  Luckily.  The next step is to expand on that idea.