Trigonometric Functions

The back cover says:
This study aid is to help the student to master the basic techniques of solving difficult problems in trigonometry. The book contains theoretical material, many worked competition problems, and some problems to be solved independently (the answers being at end of the book.) Intended for high-school and precollege students.

The book has some interesting things, for example it introduces the trigonometric functions as a feature of a trigonometric circle instead of a right-angled triangle as many books do. Also the second chapter (Identical Transformations of Trigonometric Expressions) will help you understand many combination formulae (which we learn by heart in many cases without understanding). Chapter 4 (Investigating Trigonometric Functions) treats the trigonometric functions graphically and analyzes the meaning of graphs, also with help of their derivatives.