In five years, Grant Sabatier of "Millennial Money" went from having $2.26 in his bank account to $1 million, thanks to a side hustle that he turned into a lucrative consulting company and to smart investing. "As soon as I began this journey, I looked up the best-reviewed personal finance books on Amazon," the now 31-year-old tells CNBC. Since, he's read over 360 personal finance books and "the best book on money, period," happens to be the first one he picked up: "Your Money or Your Life," by Vicki Robin and Joe Dominguez. It changed his relationship with money and his approach to spending and saving, he says: "The premise of it is that you exchange your time for money. And when you start thinking about how many hours of your life it took to save up the money to buy something, you really start thinking twice about your purchases."

Grant of Millennial Money went from broke to seven figures in a few years Courtesy of Millennial Money

For example, "Say I work eight hours a day and after taxes, make $10 an hour, meaning I'm earning $80 a day. I want to go out for a nice dinner on Friday and that costs about $80, meaning I spent an entire day of my life working for this meal. And then you start thinking about even larger purchases, like a $1,000 TV, and you think, 'How much of my life did I trade for this? Is it worth it?'" He's not the only self-made millionaire who recommends the read. Chris Reining, 38, who buckled down on his finances in his late 20s and crossed the $1 million threshold at age 35, calls it "the book that changed my life" on his blog. It introduced him to the idea of tracking his income, spending and investments in a spreadsheet and graphing his "cross-over point": when monthly investment income crosses above monthly expenses. "When this point happens is when you can seriously consider retiring because you are officially financially independent!" he explains. "No more income is needed from a job."

Chris Reining Holly Whittlef