The oldest book written in Mongolian is a biography of Genghis Khan, who created the Mongol Empire. Nobody knows who wrote the Secret History, but whoever it was, they wrote it not long after Genghis Khan’s death in 1227 AD. Most of what we know about Genghis Khan and the early Mongol Empire comes from this book.
The Secret History tells how Genghis Khan grew up, how he was enslaved as a boy but escaped, and then how he conquered his rivals among the Mongols to become Khan. Then it tells how Genghis Khan organized his empire and conquered the Uighurs. Finally, Genghis Khan conquers the Islamic empire and parts of China before he dies and his son Ogedei succeeds him.
Despite the name, the Secret History was never any kind of secret. People called it the Secret History because it’s not an official, government history. It tells what really happened, not propaganda (or that’s what we’re meant to think.) During the Ming Dynasty, people who wanted to learn to read Mongolian used to read the Secret History together with translations of the Secret History into Chinese.