There's this book I was hoping to reference in a story, but I don't remember either the title or the author. It was one of those "what if history had gone differently?" kind of novels. In this one, it was basically "What if the Black Plague had killed off a huge amount more people than it did, and America wasn't rediscovered by the Europeans until much later?" In it, the Aztecs had this thriving industrial civilization with public transportation and stuff. Unfortunetely, I read this book so long ago that I can't remember any more about it than that.
Any help is appreciated.
EDIT: Yay! The Gate of Worlds by Robert Silverberg! Thanks all!