Whether you call it "The Power of Positive Thinking", "The Secret", "The Law of Attraction", or even "Affirmations" (another term used for more or less the same thing), the message is the same: positivity is better than negativity. But it is severely overstated when assertions are made that you can "direct" your own healing, or financial circumstances.
New medicines are tested with double-blind studies including both the new medicine and placebos. Every participant believes to the same degree that they're receiving the new medicine. If "belief" were enough to receive the curative effects (as asserted), results would reflect belief and there would not be a statistical difference between the placebos and the new medicines. And we know that's simply not the case: the medicines are a critical contributor.
And millions of people believe that one cannot contract AIDS from sexual relations, or that AIDS will be cured by having sex with a virgin. Again, belief does not make it so. And telling people that belief is sufficient can be extremely dangerous.
Just as believing that you'll win (or have won) the lottery doesn't make it so: sure, the winner will claim that they wanted to win (duh!). But the losers aren't failing because of their belief.
Believing you'll get a good grade on an essay is no substitute for writing the essay.
If the book said that one should believe in God to deliver all these earthly blessings, it would be easy to see that that is not scripturally sound and millions would mock it. By substituting belief in God with belief in Self, the soundness of the assertions is not improved.
That's not to say that what you say & do doesn't have an effect on those around you: if you withdraw from others (or complain all the time), they'll tend to let you be; if you approach others, they're more likely to offer you a more active role in their own lives.