The Mongols did have their own recorded history , "The Secret History of the Mongols" (see, for example, the adaptation by Paul Kahn). It was meant for members of the various Chinggissid dynasties. Most current adaptations are from the one written by the Yuan Chinese (Kubilai Quan). There was, apparently, a different one done (not yet found) for the Il-Khanate of Persia (Hulegu's appanage). The Secret History traces the Mongols back to their mythical origins, but concentrates on the life of Temuchin, and his son Uggedai. It gives a fascinating insight into the Quan's life, and Mongol society.
The Mongols "borrowed" their written languge from the Uighers, a Turkish tribe they conquered in western Mongolia. Before that they had no written language of their own.
Jamuka was more than Temuchin's friend, he was his Anda, a blood brother, adopted in a ceremony. Their eventual rivalry,and estrangement was caused, in part, according to the Secret Hisory, by Bortei and Temuchin's mother, Houloun; an example of the power and influence wielded by Mongol women (See also Toreghine's regency after Uggedai's death, and Sorghetani Beki's maneuvering to take the Supreme Khanate from the House of Uggedai, and pass it to her son Mongke, son of Temuchin's youngest son, Tolui.
Temuchin's rise brought to his side many of the generals who expanded Mongol power during and after his death, such as Muhuli, Sabotai and Jebe Noyon (the latter two were not Mongol by birth). It also led to the destruction of the Merkits, the Tadijuts, the Tartars, and eventually, the Kerait and Jamuka.
Thanks, PzLdr, for stopping in with additional about the Mongols.