I was finished the book by the time it came around to me, and looking for something else to read.
After that, I was insatiable. I read everything I could. Picture books and such at first, and then there were books with talking rats, and books about kids who ran away from home and lived in a museum, and books about secret gardens.
Mind you, this is children's fiction, not science fiction or fantasy or speculative fiction or whatever the cool kids are calling it this generation. Children's fiction can be about talking rats and secret gardens because kids are imaginative little buggers who shouldn't be burdened with Serious Fiction. And as I got older, I read hundreds of millions of Tom Swift books, and Hardy Boys books, and Alfred Hitchcock and the Three Investigators books. Even some Nancy Drew books when the others ran out. I also read biographies, and books about dinosaurs.
4th grade ended, and we packed up and went to Europe for six months. My father was part of a program that brought American students to study in Europe over the summer, and we were based in Graz, Austria. We lived in one of the dorms (where my little brother liked to crawl through the railing of the stairs at the top of the 6th floor and dangle there, until the day he got his head stuck between the bars), and I learned how to play pinball, and we'd break into the gym through the windows 20+ feet above the gym floor and leap out to the climbing rope that dangled about 5-6 feet away. Miraculously, none of us died.
Even with all of that, I was bored bored bored, because I could only bring so many books in my luggage, and they wore out fast.
Then I got access to the school library, which had a whole bookcase dedicated to English language books. And there I muddled through incomprehensibly boring (to the sophisticated mind of a soon-to-be-5th-grader) books like The Sun Also Rises and For Whom the Bell Tolls.
But that is also where I encountered my first "real" speculative fiction: Ursula Le Guin's Earthsea trilogy. It was... mind-blowing. Everything else I'd read that included magic and fairy tales and talking rats had really been stories to amuse kids. Uncomplicated stories. Here was a morally complex tale that wasn't written like a Sesame Street sketch. A story that engaged and amazed, but also required work, required thought and reflection, and wasn't about All That Stuff That My Parents Watched On NBC News at 11. It was fantastic and real at the same time.
So, that was my first Fantasy book. Or books: I read all three.
My first SF came shortly after. My father took a trip to Vienna, where there was an English Language bookstore. He grabbed a bunch of books at random that he thought I might like. The first one I read was called Waldo & Magic, Inc., by Robert Heinlein. I found parts of it intriguing, and much of it confusing--I think that perhaps I didn't quite fully grok what a "short story anthology" was--but it left some vivid impressions, and I wanted more.
And somewhere along the line, I decided that I wanted to write some of this stuff.
So, what about you? What was the first SF/F book you encountered?