When a seat falls vacant through death or Scandal?

“I needed it to check certain abstruse points. And in there I came across the phrase ‘a casual vacancy.’ Meaning, when a seat falls vacant through death or scandal. And immediately I knew that that was the title.”

end quote by J.K. Rowling from:

15 fun facts about J.K. Rowling 

So, what I get from this is that J.K. Rowling's book is definitely a mystery and since the book also contains:

begin quote:

2. The Casual Vacancy is not Harry Potter: Words featured in Rowling’s post-Potter effort include “cleavage,” “balls” and “vagina.” end quote also from:

15 fun facts about J.K. Rowling 

So, what I would guess from all this is that the book is a mystery of some sort and not a "Children's or even a young adults book" and likely something about being responsible in a world that somehow seems a whole lot less responsible in traditional ways and more responsible in what I would call eccentric ways that are a disconnection from the way things were just before and after World War II.

But then again this is just a guess from the information given in the above article.