While traveling in India and Nepal I spent an extended time in researching the folklore around Gesar Ling through interviews with his direct descendents and his reincarnation (Namkha Rinpoche) in Kathmandu. Alexandra David-Neel's book "The Super Human Life of Gesar of Ling" proved to be extremely valuable, and fairly accurate with respect to the information I gathered.
Despite David-Neel's association with Madame Bolovsky's "Theosophical Society" this book still retains a great deal of credibility. The introduction details an encounter she had with an emissary of Gesar Ling which seems to be derived directly from his reputation as the regional protector deity of the Dege region of Tibet.
Her easy writing style makes this book accessible to the academic and the casual reader. I enthusiastically recommend this book to anyone interested in the more obscure writings on Tibet.