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, CallItFive, 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.