The New York Times tells you where to find Harper Lee:

An awards ceremony for an essay contest on the subject of “To Kill a Mockingbird,” the occasion attracts no actor, politician or music figure. Instead, it draws someone to whom Alabamians collectively attach far more obsession: the author of the book itself, Harper Lee, who lives in the small town of Monroeville, Ala., one of the most reclusive writers in the history of American letters.

In retrospect, what surprises me is that Harper Lee stayed a one-book wonder. This often-cited 1964 interview makes it clear that she had planned to continue writing after To Kill A Mockingbird:

I hope to goodness that every novel I do gets better and better, not worse and worse. I would like, however, to do one thing, and I’ve never spoken much about it because it’s such a personal thing. I would like to leave some record of the kind of life that existed in a very small world. I hope to do this in several novels‹to chronicle something that seems to be very quickly going down the drain. This is small-town middle-class southern life as opposed to the Gothic, as opposed to Tobacco Road, as opposed to plantation life.
As you know, the South is still made up of thousands of tiny towns. There is a very definite social pattern in these towns that fascinates me. I think it is a rich social pattern. I would simply like to put down all I know about this because I believe that there is something universal in this little world, something decent to be said for it, and something to lament in its passing.
In other words all I want to be is the Jane Austen of south Alabama.