A great deal of our unhappiness is caused by us wallowing in regret about the past, or worrying about the future. Many of us spend relatively little time actually being connected to the present moment, which is ironic because the present moment is the only one we have available to us. The past has already happened and cannot be changed, and the future hasn't happened yet; we have relatively little real control over it either. So all we really have to work with is the present moment.
This book gives the clearest description I've come across of why it's important to remain connected to the present moment. It's particularly important when it comes to relationships with other people, since we're difficult to connect with when our minds are somewhere else.
There are many things that can cause us to lose this connection. Over-analysing puts us “in our heads” where we think rather than feel, and often get stuck problem-solving rather than really being with ourselves and other people. Worrying about the future or obsessing over something that happened to us in the past will do it. Fear of what other people think will stop us acting naturally, as we go into our head to work out what we think will be acceptable to others, instead of being really in the present moment.
When we are fully present, we react spontaneously to other people and circumstances based on our intuition. We lose self-consciousness and stop being so self-critical. We do what is right rather than what we think will gain us other people's approval. We are free to be ourselves.
I didn't go along with everything Tolle says, and some of his more metaphysical ideas sound like wishful thinking to me. But the basic concept behind this book is one that I think every aware person needs to grasp. If we want to be authentic and real, and the only time and place you can do that is right here, right now. Hence The Power of Now.