The Authentic Life of Billy, the Kid by Pat Garrett, not the edition I'm currently reading, but there you are with an equivalent one all the same. I guess I could also recommend the two Young Guns films as supplemental material. But yeah, after these outlaw books (two books ago, it was "ecoterrorism" A Friend of the Earth from T.C. Boyle), I'll be finally moving on to a new topic, three books from Neil Gaiman. Should be fun!