I am relatively musically untrained. I got to grade 5 on piano and have done occasional basic composition as a hobby (some pop, some "classical", some "modern classical") by ear and fingers over the years on the piano. I really wanted to get back into composing so I bought this book.
I have been using it and I'm already learning a lot. And I don't just mean from a theory point of view. It's also training my ear, and teaching me to work with sheet music, and sing from sheet music, something I've never tried to do much before.
The author's method of limiting the notes, keys and rhythms that you can use works really well to make me focus on the principle being taught. Also having empty staves under the exercises in the book helps to motivate.
This is very much a "hands on" book. To use it as part of a hobby, you'll need to put aside time every couple of days or so for working on it. I worked on it for an hour or two almost every day for 2 weeks and found it very rewarding.
Although I'm no expert, it doesn't seem to have a bias towards any particular style. I have a tendency to write stuff with a lot of "out of key" chords which modulates alot. This book seems to be suitable for learning in that vein, but also in the more traditional vein. I am looking forward to getting to the section on minimalism as well - I'm a fan of Phillip Glass!
I wouldn't say this was totally a beginners book. I would recommend having some theory knowledge first, but grade 5 seems to be easily sufficient for this book.
Anyway, overall if you want to stimulate creativity in pure music then this is a great way to get going.