The subtitle says it all: "say what you mean, mean what you say, and get what you want". The first two are okay, but that third part could be tricky: it might mean some kind of mind-games. Runion isn't talking about being manipulative, however. Entirely the opposite, in fact. It's more a case of "if you don't ask, you don't get"; many are afraid of asking directly (they are so elliptical their listener doesn't even recognise what they are saying is a request), or even asking at all.
So Runion advocates using the titular "power phrases": these are short (don't ramble), specific (say what you mean), targetted (say what you want), and aren't mean (no vitriolic, or passive-aggressive, statements). The book is enlivened with lots of good examples, and counter examples. It covers requests, questioning, apologising, dealing with criticism, anger, and disagreements. The message is very straightforward, and very clearly presented.