School Teacher and Gunman: Book Three of the Clint Mason Series — William F. Martin

San Francisco bustles with gold rush fever and excitement over the coming railroads. Into the middle of this roaring wild west arrives a young man with wits, poker skills and desires to get rich and build his own ranch and home. After nearly losing his life in a deadly fight over a card game, he's given a second chance by an elderly school teacher. As he follows his love for poker and his gun skills raise the stakes, can his determined mentor save him from falling to a faster gun?