Mary Higgins Clark is known as the "queen of suspense."
At 90, she's written 37 best-selling suspense novels and sold over 100 million copies of her books in the U.S. alone. Her latest work, "I've Got My Eyes on You," was released at the end of April and is already on The New York Times Bestsellers list.
In her more than 40 years of publishing books, Clark has earned paychecks that most writers can only dream of. After releasing her first suspense novel, "Where are the Children?" in 1975, she was granted a $3,000 advance by her publishing company Simon and Schuster. As she continued to release two new books each year, her advances grew significantly.
In 2000 she was reportedly given an advance of $64 million for five books, making her, at the time, the highest-paid female author in the world. But she still remembers how she spent her first big paycheck.
"That was when I decided I could afford to go to school," she tells CNBC Make It, "and I started at Fordham University at Lincoln Center. I graduated at age 50."
The author graduated summa cum laude from Fordham in 1979 with a bachelor's degree in philosophy, and was awarded an honorary doctorate from the school in 1998.
And though her first major expenditure may have been highly practical, she says she also splurges on a gift to herself every year.
"When I started to make really big money, I treated myself to a piece of jewelry every Christmas," she says. "I would write, 'From Mary, to Mary, with love and admiration.'"
With three daughters and two granddaughters, she knew that any jewelry she purchased would never go to waste.
Clark didn't start her career as a book author until she was in her 40s. Before then, she wrote short stories and sold them to magazines. But when magazines stopped accepting fiction pieces, the mother of five says her agent suggested that she write a book.
"I liked writing short stories, but I was still a little terrified to try a book," she says. "But I figured, 'Look, this the only way I will be published now.'"
When her husband died of a sudden heart attack in 1964, Clark found herself facing life as a single mother. She says she would wake up every morning at 5 a.m. and write until 7 a.m. before getting her children dressed and ready for school. Her first published book, "Aspire to the Heavens," was a novel about George Washington's life released in 1969. Clark says she was fascinated by Washington's relationship with his wife Martha and decided to let that be the focus of her book.
"I was so intrigued by him and I thought, 'We don't know him. We just know of him,'" she said.
Years later, a Washington family member discovered the book and republished it in 2002 as "Mount Vernon Love Story," a title she felt was less confusing to readers, many of whom thought she had originally written a prayer book.
Having authored more than 50 books over her career, Higgins is best known for her suspense novels. She's even co-authored five suspense books with her daughter Carol Higgins Clark.
Her key to having continued success, she says, even at 90, is to continue working on your craft daily.
"I don't get up at 5 a.m. anymore," she says, "but I do still write every day, because I don't want to lose the stream of it."
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