At their core all these self help books are not teaching you how to reshape your speech, but emotional intelligence. They are teaching better understanding of other and yourself. You can then use this understanding to have more fulfilling relations, or more easily manipulate others if you are less well intentioned. The line is very easy to draw, being honest is natural, you say the truth and are open to genuinely understand and build on the other person responses. On the other hand manipulating takes a very different mindset of faking being open to what others are saying.