It is not easy to answer the question: "Where did you get the idea for this [or any] book?" A book is not one idea. It is thousands.
But: What was that very first spark?
Recently–prior to the Sandy Hook tragedy–I attempted to answer that question for an interview about The Great Unexpected:
Aware that so many children are more fearful now than when I was young, I hoped to create a story in which the unexpected could be something great instead of something to be feared. Two other notions wrapped themselves around this idea. One was how closely the 'real' merges with the imaginary in the lives of young people. Another notion was the intricate connections between people–some obvious and some not. . .
And so evolved this story of two young girls: in whom the real and imaginary merge; who discover intricate connections between people and places; and who learn that the unexpected can be great.
Now, post-Sandy-Hook-tragedy, I am especially mindful of the fear so many of those students and teachers must have felt, and of the fear which the survivors continue to feel. I wish I could ease it. I hope that 'intricate connections' in their lives will sustain them; I hope that the living and the dead will unite in their minds and hearts; and I hope that one day the survivors will again be able to look forward to the unexpected.