This book, published in 1985, is definitely showing its age. It is obvious that it was written at a time when home computers and personal electronics were patched together by determined geeks and that standardization of hardware and software was something of the distant future. However, that being said, much of this book's contents are still relevant with details being given on individual filters and what makes a synthesizer produce the sounds you desired that are simply unpublished in any other individual volume.
The first five chapters are background information. The chapters on music synthesis principles, sound modification methods, and direct computer synthesis methods basically comprise a first course on music and sound synthesis in general that any beginner to the field will appreciate. You simply need to ignor the sections on microprocessors and electronic devices, due to the age of the book.
Section two of the book is about the application of microprocessors to controlling conventional analog sound-synthesizing equipment as it existed in 1985. This section is interesting primarily from a historical standpoint.
Section three, chapters 12 through 18, is the real meat of the book and the part that best stands the test of time. This section is about digital synthesis and sound modification. Chapter 12 discusses the conversion of audio to and from the digital domain. Chapters 13 through 16 have details and explanations of the z-transforms of individual filters for various musical applications that can be transferred into modern synthesis programs. Chapters 17 is less interesting to the modern reader, since it basically talks about how to run down to Radio Shack, buy the electronic parts you need, and solder together a musical microprocessor system. Chapter 18, on software, would not be valuable either if it was not for some of the algorithms on display there, even though they are in FORTRAN and assembly language.
The final section of the book talks about applications. Although the implementations are prehistoric, the block diagrams and over-all ideas presented are very interesting and still relevant.
Overall, I would give this book a five star rating for breadth and depth of content and subtract one star only because of the fact that it shows its age in the implementation sections. I highly recommend it to anyone who really wants to understand digital audio signal processing. To get the most from this book you should have some knowledge of math at the level of trigonometry and algebra, prior exposure to electronics and microprocessors, and some knowledge of music. If you need more background on digital signal processing than this book provides, I highly recommend "A Digital Signal Processing Primer : With Applications to Digital Audio and Computer Music" by Ken Steiglitz. It is a very readable introduction to DSP for people interested in audio processing applications.
This book is also a great companion to the much more modern and highly recommended "Digital Audio Signal Processing" by Udo Zolzer. Do not confuse that book with the book "Digital Audio Processing" by Doug Coulter. Coulter's book is pure garbage, even at the bargain basement price at which people are selling their used copies.