I was given this book to read by a friend of mine who reads all parenting and birthing and other children related books, and I mean all of them, all she can lay her hands on :) This she recommended as an absolute must and now after reading it, I agree with her. Alfie Kohn tells not only about why punishment is wrong (even such punishment we do not call punishment, because we are those parents who do not punish their kids, right). He also tells, and more importantly in my opinion, why praise and rewarding in the classical way is equally bad for your child. Obviously he offers alternatives and also admits that pitfalls lay ahead of each parent, him including.
Read it. It is liberating.