The RenderMan Companion was my first programming book. My parents bought it for me at the Barnes and Noble bookstore where we used to go on Saturdays (this was pre-internet days for us; AOL was just arriving and would be a long-distance call for another couple of years.)
I was too young at the time to really understand the C listings in the book, but I re-read it about a dozen times and a well-worn copy sits behind me on my shelf as I'm typing this.
The thing that did stand out, that Upstill really communicated, was the concept of an API, and why standards are valuable.
Anyway, RenderMan is without a doubt the single reason I became a programmer.