William Hickling Prescott 
History of the Conquest of Mexico [PDF ebook] 
(Vol. 4/4) (Illustrated Edition)

Even though this book was written in 1890, it is surprisingly even handed in this struggle between the dominant culture of Central America and a small group of adventurers led by Hernando Cortes. The author obviously admires the achievements of the Aztecs, despite the 20, 000 human sacrifices annually made to their gods. The conditions of the Aztec nation which made it liable to collapse and the skill of Cortes are discussed. This story would not be believed if history had not instructed us it is true.
This is a magnificent epic, well worth the time it takes to read.