From the title, I expected that this would be more about manifestation and the psychology of achieving big results. Instead, it's more about emotional regulation and mastery of your mind.
The main premise is that we can most effectively control what we do in every domain by conditioning ourselves to associate certain positive emotions with actions that we want ourselves to take. He also sprinkles in lots of other wisdom about a variety of topics in life mastery, including insights on personal, financial, and relationship success.
The examples are very dated now, which made it a bit tougher to get through, and I think some of his messages are communicated better in other books now, but the core psychological principles are very valuable and worth reading.