White Desert by Loren D. Estleman
Page Murdock is a cynical, tough U.S. marshal - the last man you would want on your tail. In white Desert, Montana-based Murdock pursues a vicious gang into the snowbound Canadian north. To get his men Murdock must outwit and outmaneuver some aggressive, unpredictable Metis, but even before he approaches the quarry, the marshal runs into a colony of escaped slaves, and a band of Sioux, followers of Crazy Horse who fled to his Grandmother's Land for safety; and neither group is entirely happy to see him...