West of Eden (Eden Trilogy) by Harry Harrison.
An alternate history where reptiles evolved into an intelligent, technological species, the Yilanè, millions of years before the evolution of intelligent hominids. They are the dominant species across the planet, except still frozen North America which is too cold for the cold blooded Yilanè.
As the North American ice age retreats and the continent warms, the reptiles start to colonize it, much as the Spaniards did in our history, setting a stage for a clash between two intelligent species: one very advanced, one still in the stone age, each with radically different approaches to manipulating the world (technology).
Marketing copy quoted below. The Wikipedia entry linked above has more accurate details, but you may want to skip it to avoid the spoilers.
Sixty-five million years ago, a disastrous cataclysm eliminated three-quarters of all life on Earth. Overnight, the age of dinosaurs ended. The age of mammals had begun.
But what if history had happened differently? What if the reptiles had
survived to evolve intelligent life?
In West of Eden, bestselling author Harry Harrison has created a rich,
dramatic saga of a world where the descendents of the dinosaurs
struggled with a clan of humans in a battle for survival.
Here is the story of Kerrick, a young hunter who grows to manhood
among the dinosaurs, escaping at last to rejoin his own kind. His
knowledge of their strange customs makes him the humans' leader...and
the dinosaurs' greatest enemy.