For those who missed the review in the latest Spin-Off, or who want a second opinion...
Carol Huebscher Rhoades wrote "I would describe the ideal reader as someone who wants to develop her spinning skills by starting with a clear understanding of the processes and characteristics of yarn production."
I don't think so. I think the book has been written NOT for spinners, but for textile artists. Meaning for people who work with fabric and want to have a rough idea what yarn is - not for people who want to make yarn.
As a spinner you find much more detailed information on twist angle and tpi in "Essentials of Yarn Design for Handspinners", more detailed information on natural fibres in practically any book on spinning, and more detailed instructions for novelty yarns in "Designer Yarns". The only extra "The Yarn Book" offers is a (very) brief glimpse into the world of man-made fibres and machine processing.
The book is full of very pretty pictures, including good close-up shots (which unfortunately sometimes show several types of yarn and you have to guess to which one the caption applies), the drawings are clear enough, but I'm missing detailed technical information like chemical formulas (for the man-made fibres) or numbers.
If you want something approaching a complete spinning library, you'll probably buy the book (for the simple reason that there's so few on the market), but knowledge-wise I don't think you are missing anything if you don't have it.