From memory, much of its advice could be summarized with "act with humility, treat people well, and show an interest in what others have to say, and they will generally like you and be willing to do what you want". It had a lot of anecdotes from people describing how they handled difficult confrontations with others as part of their jobs at usually small businesses. Notoriously annoying customers would reflect on their behavior and change it after someone hears out their complaints and offers to accommodate them.

It is manipulative and can be used maliciously, but it's also just generally good advice to prevent and de-escalate conflicts. I don't think it's any more evil than a hammer is for its potential to harm people as well as build things.