First, R.I.P. Harper Lee.
Now for the controversial part. I used to get in quite a bit of hot water with some of my fellow teachers when I'd tell them my thoughts regarding TKAM. I think the book was written by Capote for the most part, maybe based on notes and memories from Lee's childhood (and Capote's, too).
At the very least, the book appears to have been heavily edited by Capote, much of it probably re-written by him. Having read other books and short stories by him, the voice and style are unmistakable to me.
My theory is that TKAM represents Capote's legacy to Lee and her heirs, something he knew would provide for her long after his death.
More than a few teachers of literature find the above theory blasphemous, but I'm just devious enough to believe it.
Regardless of the true authorship, both the book and the movie are great works, full of wisdom and wonder.