Comanche Moon by Larry McMurtry

Comanche Moon by Larry McMurtry was a pretty good prequel to Lonesome dove.  In McCraw and Call deal with Buffalo Hump , blue Duck and  mexican Bandit that captures their captain and toture him.  They are sent on the mission to find him.   Maggie gets pregnant and has Newt but Call won't claim him as his son.  The civil War happens during this time but it doesn't seem to affect Texas to much.