Kosten is one of the few chess authors I actually enjoy reading. He usually presents quality work with original ideas and analysis. This book has some good explanations about the positional themes within the Nimzo, but it is not meant as reference work or as a place to fill out your repertoire. Of course, it IS a nice place to start if you want to understand why theory develops the way it does it certain lines. Basically, what are the two sides fighting for within the pertinent lines. From a pedogogical standpoint, I would recommend it to players under 2000 who want to explore the ideas behind the Nimzo rather than trying to memorize the moves or basic themes.
However, as another reviewer mentioned, the binding on this book is bad. The pages will slowly (but surely) fall out as time goes on. And no, I haven't read it 20 times or something. Also, if you try to lie it flat (so you use your hands to move pieces on a board), you'll find that the binding cracks under even slight pressure. It might seem weird to some of you, but I would not recommend buying this book because it is so poorly manufactured. It is frustrating.
However, if you see it in a library or can borrow from a friend (who won't freak out if some pages go AWOL) then go for it. Understanding structure and how to plan based on it is one of the biggest differences between strong and weak players. This book can help you play the Nimzo better once your theoretical knowledge runs out.