On one level it's a commentary of Romans, and a very good one. It's devotional rather than verse-by-verse, and it helped me see the meaning of what I'd always seen as a very difficult book. On another, it uses the structure of the Epistle to the Romans to discuss the normal Christian life of the title.
I'd recommend this book to anyone, but most of all to Christians wondering if this is all there is. Too many "inspirational" books leave the reader frustrated at not being a super-Christian like the author. This one doesn't.