The Lawless Border — Allan Vaughan Elston

It was just one year ago that the O'Hara brothers planned to buy themselves a ranch, settle down, and raise some stock. But that was before Milton disappeared the night after he won forty-four hundred dollars at a poker game. Certain that his brother was hijacked and murdered for his winnings, Lynn vows to investigate. When he arrives in Tucson, there's one man left on his list...and he finds himself face-to-face with the ugly muzzle of a six-gun!