Marvin H. Albert was a prolific and satisfying writer no matter what name he was using or what genre he was writing in. He did many standalone westerns, as well as at least one series, the one featuring a gambler named Clayburn.
In Three Rode North, three men are sent from Mexico to find Juan Ramirez, whose family had raised Clayburn in Mexico after his own family had been killed. One of the men locates Ramirez, wounds him, and has him jailed in a little town in Texas just as Clayburn arrives to meet him. Clayburn breaks him out of jail and learns that Ramirez's father has been killed by men loyal to General Otero, who's pretty much taken over the entire area. Juan wants Clayburn's help in getting revenge on Otero and putting a stop to his depredations. Clayburn wants revenge against whoever killed the elder Ramirez, who was like a father to Clayburn.
That's the starter plot. It turns out the a man named Dietrich is in charge of Otero's army and is thinking about taking over for himself. And it turns out that Juan was in the U. S. to buy guns, which he can't afford but which he can pay for if he can break into Otero's impregnable fortress, get into his heavily guarded treasury, and rob it.
So there's that plot. Clayburn decides to go undercover, join Otero, and find out how to rob the treasury. All this in around 150 pages, so things move fast.
A lot of Albert's westerns were filmed, and I'm surprised this one wasn't. It has all the ingredients. The illustration is the first Dell printing. It was later republished by Fawcett uner Albert's on name. Check it out under either name when you're looking for a good time.