Mastering Mathematics Smartly
by Wee Wen Shih
A unique self-help website that provides comprehensive coverage of mathematics at A-level & beyond, written in a student-friendly style.
Trigonometry
This 12-page document highlights the essentials for trigonometry.
This link gives nice details about the R-sine, R-cosine forms.
Here is another link that explains inverse trigonometric concepts well.
Online book chapters (link 1, link 2) on trigonometric concepts, complete with examples and exercises.
