I really cannot tell you how good this book is. It fills a gap in the market and presents a much-maligned (not to mention neglected) aspect of Christianity in a light that is both profound and inspiring.
If you are not a Christian but are searching and are genuinely interested in what the Bible really teaches on conversion then this book is really a must-read. Likewise, if you're a Christian this book can be of much use as spiritual food, particularly in strengthening one's assurance of salvation.
A final caveat: this book is devoid of a discussion of the Cross. It is not about the reason for all conversions (justification by faith), rather it concerns the mechanics of individual conversion. That latter term is perhaps unflattering - the new birth is much more than mechanics, there is a glory and a grandeur in it that should not be overlooked - though, I'm sure you know what I mean. If you are interested in the Cross (as well, I suppose, you should be) I'd refer you to John Stott's seminal work The Cross of Christ or, for an easier read, the sermons of Martyn Lloyd Jones on the subject (compiled in The Cross).
Now don't get the wrong idea about this. Piper is in no way denying the Cross, nor indeed would he even think of such a thing. It is just that this book is, as I have said, exclusively about one specific aspect of salvation.
All in all, then, I commend this book highly and think it would be well worth your time and money.