Death on Treasure Trail — Brett Halliday

The Rio Kid saves a young boy's life in this thrilling tale of the Wild West

After three years in exile in Mexico, the Rio Kid slipped back into the United States, evading the clutches of a Texas Ranger, a crooked rancher, and a gang of desperadoes. Now he's ready to press on to Arizona to clear his name and find his fortune. There may be a $10,000 price on his head, but the Kid is unafraid -- he has his gun, his horse, and his freedom.
 
When a scream splits the black Texas night, the outlaw rides toward it. In a godforsaken cave, four men are beating a terrified young boy within an inch of his life. The Kid draws two .45s and steps into the line of fire. His homecoming can wait -- justice comes first.
 
Death on Treasure Trail is the 3rd book in the Rio Kid Adventures, but you may enjoy reading the series in any order.