The fundamental message of the book is quite simple, and counter intuitive. It's that the root of all worries and sufferings is in the way our mind operates - in that it dwells too much in the past and the future, and very little in the present.
Now, we have heard that before, but what sets this work apart is the solution that it suggests.
It is - to stop thinking (yes literally) and you will see that your focus automatically shifts to the present. In the book, Eckhart Tolle argues that the present is all you have - the past and future being mere illusions. And that to operate in the present, you don't need to think. You just need to "be".
To actually stop thinking, might seem like an extreme thing to do. And I haven't succeeded in doing that, though I've succeeded to see a lot of logic in the theory - if you watch your mind, there is indeed a lot of unnecessary activity that does nothing but wastes your processor cycles:)
Anyway, I suggest you read the book yourself before you judge; so pick up the Power of Now by Eckhart Tolle the next time you visit a book shop, it could change the way you use your mind, in a very positive way.