The Donner Party was a group of California-bound American settlers caught up in the "westering fever" of the 1840s. After becoming snowbound in the Sierra Nevada in the winter of 1846–1847, some of the emigrants resorted to cannibalism. Although this aspect of the tragedy has become synonymous with the Donner Party in the popular imagination, it actually was a minor part of the episode.
The author was about 4 at the time. The first part of the book accounts the tragic journey and rescue attempts; the last half are reminiscences of the child orphan, passed from family to family while growing up. (Summary from Wikipedia & Tricia G)