The Snow Leopard by Peter Matthiessen (1978) — not set in Qinghai but essential reading for the inner Tibetan plateau world. Matthiessen's account of a trek to the Crystal Mountain in Nepal captures the landscape and the Buddhist worldview that pervades the region around Xining with more force than any guidebook description.
Tibet: A History by Sam van Schaik (2011) — a scholarly but readable history from the earliest records to the present, with particular strength on the religious developments that produced the tradition you'll encounter in Kumbum and the surrounding monasteries. Good preparation for understanding what you're seeing.
China Road by Rob Gifford (2007) — a journey along Route 312 from Shanghai to the Kazakh border, passing through Qinghai. The sections on Xining and the plateau give a vivid sense of the city from an outsider's perspective and the transformation China was undergoing in the years before the Beijing Olympics.
The Tao Te Ching by Laozi — not because it's set anywhere near Xining, but because the caves of the Northern Mountain Temple are mixed Buddhist and Daoist, and reading the Tao Te Ching in the shadow of those cliff temples changes both the text and the place.