Nope. The secret that Napoleon Hill wants you to discover in his book is DESIRE.
I first wondered what it was when I read Think & Grow Rich the first time
in 1987. Now it's 2001 and I reread Think and
Grow Rich. Hill says that "the secret" is in the book "no fewer than 100 times." So I realized that it must be on almost every
other page. I started scanning pages and it finally hit me! The secret is "DESIRE." The concept that only when we really, strongly desire something, with "white hot intensity" will we do the things necessary to make it happen. Hill says that we must cultivate a burning desire for what we want to get. Yes, the word DESIRE, or a paragraph about it, does appear at least 100 times in the book.
This, of course, is not "The Strangest Secret", which Earl Nightingale entitled the 6 word phrase he lifted out of "Think and Grow Rich" (The Strangest Secret is "We become what we think about").

> I had been wondering about the "money
> making secret", too, when I got my
> hands on Earl Nightingale's book
> "Greatest Discovery, The Strangest
> Secret Revisited".

> In it, Nightingale, says on pages 5 and 6:

> "My reading and search for the secret
> of success continued without letup.
> ..."

> "While reading, it dawned on me that I
> had been reading the same truth over and
> over again for many years. ..."

> "The astonishing truth [is] that WE
> BECOME WHAT WE THINK ABOUT. ..."

> "Those six simple words, in that order,
> revolutionized my life. I had found them,
> clearly spelled out, just as I had so
> desperately hoped as a child of twelve to
> find them, not in some hoary, ancient tome,
> but in a book published in 1937 entitled
> Think and Grow Rich by a man named Napoleon
> Hill."

> [End of quote]

> So, Nightingale's "version" of the
> secret is WE BECOME WHAT WE THINK ABOUT. In
> reading over the "Greatest
> Discovery" again, I find so many
> principles spelled out there that originate
> from Hill that I'd almost call it
> plagiarism. I must admit, however, that
> Nightingale is an incomparably better writer
> than Hill.

> So what do you think of WE BECOME WHAT WE
> THINK ABOUT?

> Cheers,

> Wolfgang