I have no idea what the first non-children's book I read might be. I think it's either Gone with the Wind or, possibly, Anatomy of a Murder. But it might be Peyton Place, which was a real turn-on when I was 13. MY mother, whose child-rearing was passive-aggressive, pretty much abandoned me when Macbeth was born two years later. She seemed to only make contact with me when I was sick or school demanded her attention. I pretty much grew up with the only real adult supervision from my Aunt Jean, about whom I had a strange dream involving the military, a bath, and a huge monster terrifying the city.
I'm guessing Gone with the Wind; I still remember wailing like a banshee over Melanie's death.