The Origins of The Secret

In The Secret, Rhonda Byrne, in the introduction writes:

I'd been given the glimpse of a Great Secret - The Secret to life. The glimpse came in a hundred-year old book, given to me by my daughter Hayley.

Does any one know the name of this "hundred-year old book"?

I find it kind of strange that this book (or for that matter, its author) is not acknowledged.

There are, of course, four possibilities. The section on Biographies at the back of the book, The Secret are 4 dead people (the rest are alive and kicking!). These are:

Genevieve Behrend, who wrote Your Invisible Power and Attaining Your Heart's Desire
Charles Haanel, who wrote The Master Key System (believed to be the secret of Bill Gates's riches - aaah! Now you are interested! Aren't you?)
Prentice Mulford, who wrote Thoughts Are Things and The White Cross Library
Wallace Wattles, who wrote The Science of Getting Rich

So, which one is it?
I think, not explicitly acknowledging the author of the book that is at the root of the phenomenal success of The Secret is ingratitude. Wouldn't you say?