What's your book about?

You know how the most common question a writer is supposed to get is:  Where do you get your ideas?

I actually don't mind this question because I can usually answer it, more or less.  Pretty often I actually do know what image sparked a book, what secondary plotline in somebody else's book I borrowed to create a major plot in mine, what minor character in ditto led to a protagonist of mine. I don't even have to refer to Schenectady, usually. 

But in fact, this question is not at all as common a...