This book was well worth the money! I was struggling to find someone who could explain to me what all the 'letters' mean in speaker design, and how that translates into a specific size of box.
The author of this book takes great care to fully explain the meaning of the different elements and how they all relate to each other.
The only negative point I can make is that this book only focuses on Closed-Box and Ported speaker designs. Other types (Band-pass, Horn Loaded, Transmission Line, etc) are mentioned, but there's no real information to help you design one.
Given that 99% of speakers are either Closed-Box or Ported, this is not really a big issue.