Recently, I purchased both "Fit" and "Anatomy Without a Scalpel," both by Dr. Lon Kilgore. I'll try to do a full write up on both, however the content in these books is extreme. Quickly:
Anatomy Without a Scalpel - Lon writes up a complete explanation of the bones/musculature of the body. What's great about the book is it explains how these parts work while exercising. For any coach/athlete the book will help is spotting errors, possible physical flaws, and help find ways to improve. I highly recommend everybody get this. It's one thing to know how to train, it's entirely different to know how to explain why shit works.
Fit - This book is insane. It covers, to be blunt, how to get fit. No bullshit, no quick tricks. Just a break down of how to increase strength, conditioning, and anything related to getting fit. What's more impressive is the explanation of why certain activities are betting for getting fit than others. To top it off, everything is cited. The entirety of the book is backed by research. While you may not need this if you already have a set routine, you should buy this if you're looking to improve at all.
If I had Fit when I started workout out back when I was 20, I would be a monster. If I had Anatomy back when I started coaching form, I'd have been able to explain issues much better. Books are easily worth the 25 a piece.