I've read a number of these types of books and this is easily one of the best.
It begins by explaining exactly what power phrases are: short and specific targeted messages, and then warns you about using those words and phrases that dilute your message, eg. I could be wrong but, Sorry to bother you but, I was wondering, You may not like this idea but, etc.
I recognised myself saying those things, and this was the first time I'd actually stopped to think about what I was actually SAYING. So although those phrases had nothing to do with the message I was conveying, they robbed the point I was trying to get over. This book explains all of this much better than I can, and gives plenty of examples.
Much of the advice is about how to be assertive, but whilst other books talk about being assertive, this book explains how to actually sound assertive.