Jammalamadaka, R. S. and Sengupta, A. (2001). Topics in circular statistics. World Scientific.
Jammalamadaka, S. R. and Sarma, Y. R. (1988). A correlation coefficient for angular variables. Statistical Theory and Data Analysis, 2:349--364.
Mardia, K. V. and Jupp, P. E. (2000). Directional statistics. Chicester: John Wiley & Sons.