Ayres, Frank Jr., and Robert E. Moyer, Theory and Problems of Trigonometry, 2nd ed. New York: Schaum's Outline Series, McGraw-Hill, Inc., 1990.
Bittinger, Marvin L, and Mervin L. Keedy, Trigonometry: A Programmed Text. New York: Holt, Rinehart, and Winston, Inc., 1969.
Cole, J. P., and Frank Rickey, Plane Trigonometry, 3rd ed. New York: Holt, Rinehart, and Winston, Inc., 1969.
Swokowski, Earl, Fundamentals of Trigonometry. Boston: Prindle, Weber & Schmidt, Inc., 1968.