The Other Time, by Mack Reynolds and Dean Ing (epub/Kindle/pdf)

A step in an odd direction -- a moment of dizziness - and archaeologist Dan Fielding was thrust through an invisible barrier found hundred years into the past. He was still in the Mexican desert -- but it was the desert of the 16th century, and Mexico was in the grip of the conquistador Hernando Cortez.

Inevitably, Cortez captured Fielding -- and learned of the rich territory north of the Rio Grande. The land that would one day become the U.S. would be his next conquest. Unless Fielding could rally the Indians and erase Cortez's bloody footsteps from the New World forever!