Let's get three gripes out of the way first.
Firstly, the title of this book is overgeneral. It should more accurately be called "How evangelical Christians change", because it's written unashamedly from within that world view: it's assumed that the Bible is the infallible word of God in a fairly literal sense, so Paul wrote Colossians, Peter the semi-literate fisherman wrote the very polished and clearly second-century 2 Peter, and all the different voices in the Bible speak in agreement, no matter what the vast majority of critical scholarship might say. Unless you subscribe to, or at least can live with, that version of reality, you might find the book hard going.
Secondly, despite being written by two people, the book disconcertingly uses the first person singular throughout, with no indication of who "I" refers to in each anecdote. Maybe it doesn't matter much, though; the back cover reveals both authors to be middle-aged gentlemen wearing dark jackets and open-neck shirts, each with a wife, four children, an M.Div and a D.Min. Indeed their only obvious distinguishing feature is that one has glasses and a moustache and the other one doesn't. I even wondered if those adornments are just a disguise, and it's really one person pretending to be two in order to get a better deal from the publishers; but then that person would have to have two wives, which would be a definite no-no, despite the many Old Testament precedents.
Thirdly, and most bizarrely, one of the items on their list of types of "Christian externalism" -- superficial forms of faith that don't lead to real change -- is something they call "mysticism", by which they seem to mean some kind of addiction to superficial emotional highs. I don't think Evelyn Underhill or Thomas Merton would recognize that use of the term, and it's a shame, because the unsuspecting reader might be put off it altogether, and the whole point of mysticism (properly understood) is that it's a process of deep transformation of the personality from within -- just what this book is about, in fact.
So what's good?
Well, in the end, none of the above criticisms are that important (especially the one about the shirts). What the authors do superbly well is to bring an interpretation of Christianity based largely on the more gnostic regions of the New Testament -- the (pseudo-) Pauline letters, especially Ephesians and Colossians, and the gospel of John -- to bear on the thorny problems of living in the real world: relationships, money, addictions, work, pressures of all kinds. In their view, true change can only come through a relationship with Christ actively pursued and developed; mere cognitive or emotional adjustment just doesn't go deep enough.
The stress throughout is on what's going on within us: we are responsible for how we respond to the heat of life's trials and blessings, for whether we yield thorns or fruit in response to them. But we're not on our own: the cross, and the new identity it opens up to us in Christ, makes all the difference because the power of Christ to live his life through us is what really enables change. This "heat-thorns-cross-fruit" model forms the backbone of the book, and its real strength is the way the authors combine it with their obviously extensive experience of counselling to produce insights that are direct and often extremely challenging, especially in their demands for honesty with oneself and others, while never leaving the reader feeling hopeless. It could be described as a practical application of the writings of Watchman Nee.
In summary: if you believe in the reality of Christ but are impatient with how little that reality seems to impact who you are and how you live from day to day, read this book and do what it says. You will be in good hands.