Isabella Bird traveled by horseback from Truckee, California, through the Tahoe Basin and on to Colorado where, during the autumn and early winter of 1873, she explored more than eight hundred miles of Rocky Mountain terrain only recently opened to pioneer settlement. Riding not sidesaddle but frontwards like a man (though she threatened to sue the Times for saying she dressed like one), she encountered magnificent unspoiled landscapes and abundant wildlife-including rattlesnakes, wolves, pumas, and grizzly bears.

In letters to her sister, first printed in the magazine The Leisure Hour, Bird recounted her adventures and her impressions of the small remote townships and the miners and pioneer settlers she came across. For a time she was joined by Jim Nugent, "Rocky Mountain Jim," an outlaw with one eye and an affinity for violence and poetry and someone Bird described as "a man any woman might love, but no sane woman would marry," in a section excised from her letters before their publication.

A Lady's Life in the Rocky Mountains, Bird's fourth and most famous book, remains a classic of Western literature.