TOD: I have no memory of what the first "adult" novel was that I read. I am certain it wasn't too adult.
It may have been "The Prince and the Pauper" by Mark Twain. I love to read, and I have done so since I first began to read.
The first book I remember that was very serious was "Cry, the Beloved Country." It took me a while to get into. and I remember going through it rather slowly. Happily, I recall I looked the title up in an encyclopedia and read an overview of Alan Paton's point of view. It was the first analysis I had ever read. It was my first inkling that there were people who pored over meaning and motivation about stories.