Spatial covariance models

Bertil Matern; Spatial Variation. Medd. Statens Skogsforskningsinst.49: 5, Ch 2-3.
(Reprinted in Springer Lecture Notes In Statistics, vol. 36)

Cressie: ch. 2.3.1, 2.4, 2.6.

Richard H. Jones: Stochastic processes on a sphere. Annals Math. Statist. 34: 213-218.