Mormon Country by Wallace Stegner (1942) is the place to start. Stegner grew up in the American West and writes about the Mormon settlements of Utah — including the Sanpete Valley communities — with deep sympathy and literary authority. It remains the definitive account of what this part of the country is and how it came to be.
Desert Solitaire by Edward Abbey (1968) is set further south, in the canyon country around Moab, but it captures the broader Utah desert experience — solitude, scale, the politics of public land — better than almost anything else written about the region. A useful counterpoint to Stegner's more settled perspective.
Refuge: An Unnatural History of Family and Place by Terry Tempest Williams (1991) is set largely around the Great Salt Lake, but Williams is one of Utah's finest nature writers and the book's meditation on landscape, loss and the natural world resonates throughout the state.
Utah: A History by Charles S. Peterson (1977), part of the States and the Nation series, provides a compact and readable overview of Utah's political and cultural history from the earliest settlements through the 20th century — useful context for understanding the Mormon farming towns of the Sanpete Valley.