^Charles Bell, Tibet Past and Present, Motilal Banarsidass Publ., 1992, 376 pages, pp. xviii and 78–79: "Slavery was not unknown in the Chumbi Valley during our occupation, but proximity to British India had greatly lessened the numbers of the slaves, so that only a dozen or two remained. Across the frontier in Bhutan there were a great many. / Slaves were sometimes stolen, when small children, from their parents. Or the father and mother being too poor to support their child, would sell it to a man, who paid them sho-ring, 'price of mother's milk', brought up the child and kept it, or sold it, as a slave. These children come mostly from south-eastern Tibet and the territories of the wild tribes who dwell between Tibet and Assam. / Two slaves whom I saw both appeared to have come from this tribal territory. They had been stolen from their parents when five years old, and sold in Lhasa for about seven pounds each. [...] / Slaves received food and clothing from their masters on the same scale as servants, but no pay. [...] / The slavery in the Chumpi valley was of a very mild type. If a slave was not well treated, it was easy for him to escape into Sikkim and British India."
Goldstein, Melvyn C. A History of Modern Tibet, 1913–1951: The Demise of the Lamaist State (1989), first Indian edition (1993) Munshiram Manoharlal Publishers, New Delhi, ISBN81-215-0582-8 Pagination is identical to University of California edition.