This synopsis from www.mymindsink.com does a better job than I can of simplifying the messages in the book:
"You cannot change the past, and planning to be happy in the future doesn’t work. By letting go of past experiences you cannot change, and not worrying about future events you cannot control, you eliminate most of the complexities of life. Appreciate the beauty in the moment and be happy with what you have in the present. The ability to do this, and live life in the present moment, is what the author calls The Power of Now.1
The secondary theme builds on your ability to focus on the present moment. Once you purge concern of past and future events from your thoughts you can begin to create your own reality. How you feel and react in the present moment becomes a choice. Accept the external things you cannot control and direct your internal perspective over which you have complete control. The result is peace and happiness."