As a resource for valuing antiques, and as been touched upon in the other reviews, this books fails monumentally; as with all markets, antique prices are a function of market fundamentals, and as a consequence, prices are constantly in flux.
However, this book is worth its weight in gold as a means of identifying and attributing antique furniture to specific English monarchial reigns. To the best of my knowledge, it is the most comprehensive resource for antique furniture identification that I have ever encountered. Is it a George IV or William IV bookcase? A survey of the hundreds of pictures of English furniture in this book will soon lay your question to rest, and leave you with an encyclopedic knowledge of designs, terms, and names of important cabinetmakers so that you have a good mastery of the discipline.
As always, questions regarding the valuation of antique furniture is best gleaned from your local auction house floor, and not from a book.