Nearly a century ago, journalist Napoleon Hill studied more than 500 self-made millionaires including Andrew Carnegie, Henry Ford and Charles M. Schwab. His interviews and research culminated in the 1937 bestseller, "Think and Grow Rich."

The 80-year-old personal finance classic breaks down what Hill calls the "money-making secret" in 13 steps.

Hill's wealth-building advice remains relevant today. In fact, his book is a favorite of several of today's self-made millionaires, including "Shark Tank" judges Robert Herjavec and Daymond John.

John, who turned $40 into a $6 billion clothing line, has read it 20 times. And Herjavec, who emigrated from former Yugoslavia to North America with just $20 in his pocket, says that every professional should read it. "It's epic," the self-made tech mogul tells CNBC.

"Think and Grow Rich" hones in on the idea that getting rich is fundamentally about mindset.

In fact, Hill barely mentions the words "money," "wealth" or "finances," and chooses, instead, to focus on the psychological barriers that hold many people back from building fortunes.

A third Shark, Barbara Corcoran, also endorses the best-seller. "The book teaches you: If you can imagine it, you can accomplish it," she says in "Think and Grow Rich: The Legacy," a 2017 documentary based on Hill's book. "And I found that to be entirely true. If I could imagine it, I always got it."

"You have to not only picture it," continues the real estate power player, "but picture it in living color and in great detail."

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