Down Fall River Dead - James C. Work

In the 1920s, Rocky Mountain National Park is brand new, yet there's already a feud going on between tour bus drivers and independent guides. One guide is found shot to death, but no one saw a gun. No one heard a shot. Then a bus driver is killed the same way. 

Ranger McIntyre's glamorous lady detective friend, Vi Coteau, will investigate. Her partner, Eleanor, goes undercover and gets a job driving for the bus company. When it becomes evident that the buses are hauling something other than sightseers, our daring ranger sets the most complicated and dangerous trap of his career.