High Country (Anna Pigeon Mysteries) by Nevada Barr

When four young seasonal park employees at Yosemite National Park mysteriously vanish without a trace, Anna Pigeon is sent in to work undercover at the park's historical Ahwahnee Hotel to uncover the truth, but as her investigation into a pervasive evil permeating the region progresses, she finds her own life on the line. 125,000 first printing.