Cover art
Reynolds, Mack, and Dean Ing. The Other Time. Baen 1984.

Reynolds, Mack, and Dean Ing. The Other Time
Divergence: 1519 CE
What if: The Tenochas (Aztecs) were better prepared to face Cortes.
Summary: A modern anthropologist in Mexico is displaced in time into the middle of Cortes's expedition shortly after its landing. But after one of Cortes's men tries to kill him, he flees toward Tenochtitlan. Slowly he turns from an attitude of fatalistic observation to increasing participation and manipulation of events.
Published: Simon & Schuster 1984; and Baen 1984 (0671559264).