Scott Alexander in Astral Codex Ten:
Sometimes scholars go on a search for “the historical Jesus”. They start with the Gospels, then subtract everything that seems magical or implausible, then declare whatever’s left to be the truth.
The Alexander Romance is what happens when you spend a thousand years running this process in reverse. Each generation, you make the story of Alexander the Great a little wackier. By the Middle Ages, Alexander is fighting dinosaurs and riding a chariot pulled by griffins up to Heaven.
People ate it up. The Romance stayed near the top of the best-seller lists for over a thousand years. Some people claim (without citing sources) that it was the #2 most-read book of antiquity and the Middle Ages, after only the Bible. The Koran endorses it, the Talmud embellishes it, a Mongol Khan gave it rave reviews. While historians and critics tend to use phrases like “contains nothing of historic or literary value”, this was the greatest page-turner of the ancient and medieval worlds.
More here.