"Fandango," or "Beneath A Hunter's Moon," both by Michael Zimmer.
"Fandango" is the story of a trapper's brigade leaving the Mexican settlements in the 1820s, trapping north toward the Great Salt Lake.
"Beneath A Hunter's Moon" is set in 1832, and follows a Metis buffalo hunt from the Red River Valley into what eventually becomes North Dakota.
Both are well researched, with lots of detail, and got good reviews. Richard Wheeler called "Fandango" one of the best mountain man novels ever written.