I had a science fiction class in high school. I read a great book that I would like to find and re-read, but alas I can't remember the title. I suspect it was written by Ray Bradbury, but I can't be sure. The story starts with a group of humans walking through a forest. I remember they uncovered an old Camel cigarettes package as if it were a relic. As the story unfolds, they are in a world inhabited by beings much larger than humans. They end up in a city covered by a dome, with a river running through it. The humans are slaves to the beings, and the main character rides on the being. In the end, the main character realizes the beings have weak spot, which if hit, renders them powerless for a while. The character finds the right moment, and makes his escape. Anyone know what book I read? This is driving me crazy.

Not Bradbury, but vaguely familiar.
Maybe Leslie F. Stone's "The Human Pets of Mars"?

Braling II
Thanks. I will investigate your suggestion.