What was it? For me, it was the funny papers.
I still remember many things from when I was very young. One thing I remember is having to beg one of my parents or my grandmother to read me the comics from the newspaper. I still remember thinking to myself, "I can't wait to learn how to read so I can read the funny papers all by myself."
My parents put me in a private kindergarten (back then there were no public kindergartens where we lived) when I was four years old. Our teacher was a retired elementary school teacher who had opened her own kindergarten after retiring from public school. I spent two years there before I started first grade. By the time I was five, one year before I started first grade, I was reading the funny papers all by myself. I soon graduated up to reading paperback collections of Peanuts comics that my mother had collected. I still have them, some of them the really old versions back when Charlie Brown looked much younger and didn't have quite such a large head.
So what was the first "real" book I ever read? That would be Tom Sawyer. It was a Christmas gift from a bibliophile aunt of mine who recognized a kindred spirit in me. It was daunting, such a thick novel with such grown-up language to my six-year-old eyes. At first I picked through it, reading the chapter about Tom and Huck witnessing the murder in the cemetery three or four times before I actually read the book. Even at that young age I was already attracted to the macabre. I read the whole book for the first time when I was seven (if I recall correctly). By the time I was 12 I had probably read it at least a dozen times.
That first time I read it, when I got to the part about Tom naming two of the apostles as David and Goliath, the chapter ends with, "Let us draw the curtain of charity over this scene." I remember taking it to my mother and showing her what it said, and then concluded, "That means he got a spanking." My mother laughed, but didn't try to correct me. Later on I realized that's not what it meant, but I was still pretty sure that if I had stood up in front of the entire congregation and stated that David and Goliath were two of the apostles, I would probably have been in trouble for it.
When I reached my teen years I stopped reading it, and many years went by before I even touched the book again. Revisiting Tom and Huck's adventures was like reliving parts of my own childhood, remembering how I perceived certain passages so many years before, and contrasting them with how I perceived them as an adult. It is still one of my favorite books, however, I was well into my twenties before I first read The Adventures of Huckleberry Finn.