OK, first off. If you don't want to bring your child up according to Biblical principles, you won't like this book. If you think smacking is always bad, you won't like this book. If you think children are naturally good and delightful and just need a little bit of shaping in their behaviour from time to time, you won't like this book.
But if you want to take seriously what the Bible says about raising kids, this is probably the best, most helpful and most practical book you'll come across. The big difference between this one and everything else I've read, is that Tripp deals with the heart issues, rather than just external behaviour. Most parenting books focus on discipline/reward as a way of modifying behaviour. (E.g. if you tidy your room, you can have a treat. If you refuse, you won't.) To an extent, this works; at least in the same way that you can train a dog to fetch a stick. The problem is that it doesn't deal with the question of why the child is refusing to tidy their room in the first place.
This is what sets Tripp's book above everything else I've seen. He deals with the heart-issues. Why do children misbehave? Why are they selfish and unthoughtful and uncaring? His conclusion is the same as mine: they are by nature sinners like the rest of us, and need the gospel of Jesus just like the rest of us do. They need to understand sin, forgiveness and redemption just like us grown-ups.
Tripp sees parenting as a series of opportunities to teach your children their need of the gospel, and to show them how it works in practice. Highly recommended!