I highly recommend a fictional novel by Orson Scott Card. (I know, I know, its fiction, but its still really excellent.)
The novel is called, Pastwatch: The Redemption of Christopher Columbus. it's a sci-fi novel, but if you can get past that, the author presents his ideas about how the american civilizations could have been protected from European invasions, and what would have happened if they hadn't been so completely wiped out by the Conquistadores. He vaccinates the natives, to counter the diseases; introduces a variant form of christianity, both to ease the fear of the God-like europeans and to aid in diplomatic relations later; and stalls the advance of the Europeans to the New World by about forty years. He postulates that in that forty year time span, the Aztec empire, on its last legs, would have toppled, but been replaced by an empire of Tlaxcalans, who would utilize the leftover Aztec infrastructure to unify most of the tribes who had fallen under Aztec domination. also some interesting things in there about naval tech, and its state in the Americas at the time of the conquest.
speculative, i'll grant you, but really cool.