Maybe, The Secret Garden? I think I read that around 3rd grade? Of course I read a lot of Beverly Cleary and Judy Blume, but I'm not sure if Ralph S. Mouse came before or after. Or if it counts. So - the first true grown-up adult novel that I read was Pride & Prejudice.
I read it for the first time in 5th grade. I didn't understand a word of it. All I think I got out of it was that the mother wanted to marry off all of her daughters. Everything else.... But, I read it.
Purchase Now from Amazon: Pride and Prejudice