The best one, and the one usually recommended by academics, is Paul Ratchnevsky's biography, "Genghis Khan: His Life and Legacy". There are other decent ones, but this is definitely the best.

If someone recommends Jack Weatherford's book, then be careful; it's an entertaining read but some rather dodgy history.

In addition the more modern academic sources recommended already, I would also advise trying to read on of the English translations of "The Secret History of the Mongols"; a work commissioned shortly after the great Khan's death, and one that is often used as a source to this day in books about Genghis Khan and the Mongols during that time period.

It is by far the most broad and comprehensive record we have of his early life and rise to power in Mongolia. However be prepared to take a decent chunk of I with a grain of salt. Upon reading it you will find there are some pretty obvious exaggerations and mythology mixed into it to make it a better story. And of course, since it is one of the few things we have about his early life, a lot of the stuff that isn't obviously a fabrication/exaggeration is still unverifiable. Still definitely worth a read though, if only for the insight it gives on the Mongol mindset and culture at the time.