Vad sägs om The Fireball trilogi - Fireball , Ny Found Land och Dragon Dance av John Christopher ?
In the first novel, Fireball, Simon and Brad are cousins who are mysteriously transported to an alternate history Earth, where the Roman Empire did not break up and Europe remains in pre-Dark Ages technology. In an attempt to improve their status in the new realm, Simon and Brad aid the Church, which is oppressed, to launch a coup by introducing the stirrup and the longbow. The coup succeeds, but the boys did not anticipate the Church as a state power would force everyone in the Empire to convert or die. At the end of the first book, they sail away to the New World, which in the realm, was not discovered yet by the Old World.
At the beginning of this novel, they managed to reach the American continent safely. They are received warmly enough by the native tribes in North America, but soon find themselves yearning for more advanced civilizations.
Trekking across the continent, they head for the only civilization in America which has significant urban living - the Aztecs. As the Roman Empire has persisted in Europe of this realm until the 20th century due to unchanging externalities, the Aztec too has continued without encounters from the Europeans' discovery of the New World.