$45 is cheap. A bargain. A stone bargain.
Think of it, just to read that book cover to cover is many hours of solid reading. Just reading. Not even studying. A lot of what's in there deserves serious study. His chapter on listening, how he explains how to do it, explaining the glossary of audio terminology- glare, grain, etc- this one chapter alone is how I learned to become a good listener. Before this I couldn't even tell one CDP from another! It did not happen by reading either, let me tell you! Had to read, study, think, listen, read, study, think, listen.... until one day BAM it is sinking in!
So just that one chapter is gold. All by itself. Not to mention understanding power supplies, how components work. There is right now at this very minute a guy trying to improve dynamic range with op amps. If he read Harley he would understand how a power supply works and be making progress instead of running in circles. To name just one current thing at random.
But that's not even the best part! The best part is everything in this book, you can count on it actually being true! Not interweb blather, real serious knowledge you can rely on. Invaluable.