Reddit mentions of The Secret History of the Mongols: The Life and Times of Chinggis Khan

Sure. If nothing else it's a great story. Ideally I'd recommend the translation by Igor de Rachewiltz, but it's really pricey. Urgunge Onon's is cheaper and, I hear, quite readable. Or if you can stomach King James English Arthur Waley's is even cheaper and Cleaves' is available free online (seriously though those last two translations are a chore – if you're put off by them look into one of the modern, plain English versions rather than giving up).