A great book for brushing up/re-learning pre-calc is "Precalculus Mathematics in a Nutshell"[0] by George F. Simmons (you can find it used cheap). He really boils it down to the essentials. For example here's how he opens his chapter on Trig:
"Most trigonometry textbooks have been written by people who appear to believe that the importance of the subject lies in its applications to surveying and navigation. Even though very few people become surveyors or navigators, the students who study these books are expected to undertake many lengthy calculations about the heights of flagpoles, the widths of rivers and the positions of ships at sea. The truth is that the primary importance of trigonometry lies in a completely different direction - in the mathematical description of vibrations, rotations, and periodic phenomena of all kinds, including light, sound, alternating currents and the orbits of the planets around the sun. What matters most in the subject is not making computations about triangles, but grasping the trigonometric functions as indispensable tools in science, engineering and higher mathematics. These functions and their properties are the sole subject matter of this chapter."
The entire book has that vibe. It's wonderful.
[0]https://www.amazon.com/Precalculus-Mathematics-Nutshell-Geom...
Precalculus, Coursera, https://www.coursera.org/course/precalculus
"Precalculus Mathematics In A Nutshell", George F. Simmons, http://www.amazon.com/Precalculus-Mathematics-Nutshell-Geome...
"The Hitchhiker's Guide to Calculus", Michael Spivak, http://www.amazon.com/Hitchhikers-Guide-Calculus-Michael-Spi...
