Jones has really cornered the market with this superb bible on valve audio - there really isn't a more comprehensive guide with the same refined expertise that he clearly possesses. The writing is clear and concise covering a wide range of useful topics.
If like me you are a relative beginner in amplifier theory it will provide a low-pain introduction into the field (be warned there's few introductory notes on basic electronics - you'll need another textbook for that; however, the style encouages learning a minimum of theory for maximum effect), and I've been assured by various other sources that for advanced users it will still serve as an invaluble reference book.
I'd also like to note that it would be an ideal first book for getting into musical instrument amplification.
Top marks.