Destiny of eagles / William W. Johnstone.

Enhanced descriptions from Syndetics:

Falcon MacCallister's father was a legend in the West, while Falcon's quest for justice has driven him onto the wild side of the law. Famed as a gunslinger, feared for his lethal speed and accuracy, Falcon decides to make it a fair fight when he comes upon a man being attacked by a bandit gang in the Dakota Territory. The man is Teddy Roosevelt, the Rough Rider, himself. Roosevelt knows a brave man when he sees one. And he calls on Falcon when a judge's daughter is taken by an outlaw trying to pry his brother free. With no trust in-or from-the law, Falcon has only one way to do this job: alone and ready to kill. But the outlaws know he's coming, the woman he's trying to rescue knows some tricks of her own, and winning a bloody battle in the Dakotas will take more than courage-it will take a man's will to live like a legend...or die like one.