I don't know what Rhonda Byrnes's "The Secret" is exactly but it appears that she's collected centuries of wisdom from various sources and combined it into an accessible system. I can't speak to the value of "The Secret" book or movie but I do know that I've come across what I consider to be The Secret. Here it is for free:
Control Your Thinking, Control Your Life.
THAT is the secret to human existence. I came across this "secret" while reading Charles F. Haanel's "The Master Key System." Published in 1916, this book tells you the secret on page two instead of dragging out the mystery for 200 pages.
The secret is that every human being has the ability to create what he or she wants through their thinking.
It's that simple yet incredibly hard to execute.
I highly recommend that you read "The Master Key System" and read this post from the Addicted2Success website.