Compton, Ralph. Sundown Riders.

Thorndike Press large print Western series.

"In Omaha, Tom Calvert boards a riverboat to play high-stakes poker, but accusations of cheating cause some serious trouble, and a deadly gun battle ensues. Tom is injured and knows that his enemies will be looking for him, so he reluctantly accepts a bargain from young stowaway Asher. In exchange for Calvert teaching him gunslinging skills, Asher will guide them to a possibly mythical town of peace and plenty called Friendly Field. To get there, they just have to battle assassins, dangerous Shoshone, and the rough wilderness of the Oregon Trail"--