Stegner's biography is more like two books. The first part is an adventure story retelling Powell's trip down the Colorado River in 1869. The second part is the political saga of his time in Washington, leading various bureaus and managing the scientific exploration and documentation of the western US. In the second part, Stegner is rather self-righteous about Powell as a man of science leading the battle for truth in a corrupt Washington, which almost spoils the story. However, he salvages the narrative with an engaging discussion about the battle between myth and reality in the arid lands beyond the 100th meridian. Before 1900, two-thirds of all homesteaders failed to stay on the land.
Stegner's book is worth reading for the story of Powell's trip down the Colorado, as a detailed history of the politics of land policy in the 1880s and 1890s, and perhaps for some of Stegner's discussion of Powell's geological ideas about erosion and canyon formation. However, Stegner does a poor job of considering Powell as an ethnographer. His section on why tribal names make 'un-poetic' place names is hard to read, and his review of Powell's 'classification' of tribal societies is repulsive. Overall, it is somewhat of a caricature of itself, useful if you are interested in mid-20th century writers and public voices.
Stegner's book seems more about Stegner's perception of Powell than a serious biography. Mark Bramhall was a fine narrator, with the right tone for the narrative, as if he was channeling Stegner. I would read another Stegner book if I wanted to know about his thoughts on a particular topic, but I don't read Stegner because I think he is a great writer. As Naipaul said, he read Conrad because Conrad was one of the few writers writing about places he cared about - the same goes for me.