What was the first science fiction/fantasy "novel" that you remember reading and who wrote it? I say "novel" in quotes because I want to eliminate picture books such as Dr. Seuss but include works that would appear to be novels to the kids we were when we read them. Or to put it another way, let's say "chapter book" if that makes you more comfortable. *
The earliest science fiction chapter book that I can remember reading was The Spaceship Under the Apple Tree by Louis Slobodkin, probably in the third grade (around 8 or 9 years old), from my school library.
What really got me hooked though was a few years later when my best friend would give me his Ray Bradbury short story collections during silent reading in class and tell me to read this or that story. There was no looking back at that point!
*Pratchett still counts even though most of his books don't have chapters. ;)