Its theme was the battle against "Resistance," this destructive force within us that makes us delay working on the long-term, creative projects that each of us thirsts to work on but keeps procrastinating starting. It provides some refreshingly honest and direct advice for how to battle against this force by being a "Professional," and it outlines exactly what it means by that term.
I found the descriptions of how Professionals (writers, artists, entrepreneurs) work and their rituals to be really helpful, and I have indeed seen many of the qualities it mentioned in people I consider to be very effective. I also liked the connections the book drew with classic/ancient wisdom and philosophy. I found the more spiritual/religious tones near the end of the book to be less interesting or compelling though.
Overall, this book was a quick, easy, but very powerful read, and it hit a strong chord with me. It's a good book for kicking your butt into gear if you find yourself wishing you could be doing something you're not and wondering why it's so hard to get started on the things you say you're passionate about.