Robbins' book is set in 1511 and is based on the true story of Gonzalo Guerrero, who was shipwrecked and found himself washed ashore on an unknown part of what was then the New World. He and his fellow Spanish shipmate survivors were captured and enslaved by a Mayan tribe, and Guerrero ended up embracing the Mayan culture. When one of his fellow captives escaped and joined up with the invading forces led by Cortes, Guerrero became a Mayan warlord and defended his adopted people.
According to Deadline, Emmerich has been interested in the story for years, pursuing a way to bring it to the screen. Just don't expect him jump into it as the next project after Stonewall. There's still the small matter of bringing the long gestating Independence Day (1996) sequel to our screens.