This is one of the best "How to" books I have ever read on any subject.
It works on several levels. Even though it takes a "beginners" approach to introducing many topics, it doesn't shy away from "real" music. For example, even the first simple exercise contains something different - 5/4 time. A beginner will simultaneously learn the basics of chord progressions and melody writing, stripped down to the bare essentials, while delving into modern techniques like 12-tone rows and picture music.
The key to the book is the concept of restricting the musical palette by some simple rules, to prevent the budding composer being overwhelmed by the possibilities. Right from the first page, you are working on real composition examples.
Even though the book is not particularly aimed at computer music or sequencing, electronic music enthusiasts should find it useful. A lot of the minimalist techniques are ideal for sequencing. Plus, if your musical background is pretty basic, and you are looking to broaden it with a mix of traditional and contemporary techniques, this book should take you a long way.
By its very nature, a book like this can only touch on some areas. So you will probably want to supplement it with some other material if, for example, you want a bit more detail on counterpoint. Still, it manages to cover a huge amount of ground, with the most detail where it counts most - developing and harmonising melodies, and a very good section on writing music to lyrics (ie songwriting).
One last point - a lot of theory and composition books miss the mark with contemporary musicians because they approach the subject from the purely "classical" angle of cadence and resolution. This book is soundly classical in its approach to harmonisation and melodic development, but uses the comfortable pop/jazz approach to chord progressions, so it shouldn't lose any reader.