This (as far as I know) is the first book to use the term permaculture, first published in 1978, and which has led to a vast array of books expounding on the theory and application of the concept. Mollison and Holmgren define permaculture on page 1 of this book as "an integrated, evolving system of perennial or self-perpetuating plant and animal species useful to man". They go on, through this book, to lay out the basics of the concept, qualifying their definition and expanding on this particular brand of sustainable living. It is inspiring, and practical, and it is easy to see why it has inspired so many other practitioners and authors.
This work is probably not necessary unless you are interested in the history of the concept though. I haven't compared every single picture, but it looks like most of the information in here is repeated in Mollison's follow-on Permaculture: A Designers' Manual, and the vast majority is repeated in the cheaper Introduction to Permaculture. If you are interested in the concept, great, and start with the Introduction to Permaculture. For more pictures and examples of how these concepts work in real life, check out Restoration Agriculture or The Resilient Farm and Homestead. If you decide to get the permaculture certificate, you'll need Permaculture: A Designers' Manual since that's the textbook for the course.
Great info, just repeated and expanded upon in later works, now mostly interesting for historical reasons. Save your money and pass on this one.