This 1880 paper by William Colenso includes
information on kūmara.
More suggestions and sources
Best, Elsdon. Māori agriculture: the
cultivated food plants of the natives of New
Zealand: with some account of native methods of
agriculture, its ritual and origin myths.
Wellington: Te Papa Press, 2005 (originally
published 1925).
Davidson, Janet M. The prehistory of New
Zealand. Auckland: Longman Paul, 1987.
Leach, Helen May Keedwell. 1,000 years of
gardening in New Zealand. Wellington: Reed,
1984.