After years and years of lessons, I could play just about anything, but I couldn't have told you exactly WHAT I was doing. My teachers never spent much time on theory. I could play all the scales, but had never figured out how they worked together.
When I started to teach piano on the side, I wanted to make sure my students didn't suffer from that knowledge gap. I went to find a quick overview, and this book fit the bill.
This book is CLEAR, CONCISE, and ENLIGHTENING as to how chords and scales and transposing all work together.
Most importantly, the book is well designed so it's easy to find what I'm looking for when I have a specific question. The layout is clean and uncluttered, which is a nice change from all the books that seem to delight in visual clutter.
A+. Money well spent!