Managed to wade my way through it during our vacation. My thought was: This is a Pulitzer?!
I have serious doubts that it would ever get published today with plagiarism charges running amok. From Wikipedia:
The novel is directly based on the letters of Mary Hallock Foote, later published as A Victorian Gentlewoman in the Far West.
Stegner's use of substantial passages from Foote's actual letters as the correspondence of his fictional character Susan Burling Ward was and remains controversial among some scholars. The controversy is somewhat tempered since Stegner had received permission to use Foote's writings, implying as much in the book's acknowledgements page.
Still - the acknowledgement was bare. It did not say that it was excerpted from actual letters just that the story was based on her life.