"I have never put in sex or four-letter words in the books and I have certainly gotten away with it," she added with a laugh.

That by itself is quite a feat, given the old saying that sex sells.

Her first was the suspense thriller, "Where Are The Children," originally published in 1975. More than four decades later, Higgins-Clark said she remains true to her early formula.

"I can look at my first book, and I really haven't changed my style, not at all," the prolific author told CNBC's "On The Money" in a recent interview.

Mary Higgins Clark's latest novel, "I've Got My Eyes On You," just hit the top spot on the New York Times hardcover fiction bestsellers list . At age 90, the Bronx-born author has been doing that for more than 40 years.

Known as "the Queen of Suspense", she's written more than 50 books that have sold more than 100 million copies in the U.S. alone. Higgins-Clark didn't even publish her first novel until she was well into her 40s, and went through several jobs before she first put pen to paper.

Writing, she told CNBC, is "my only talent, I can't sing, I can't sew, I can't dance."

With that many titles, what's the secret to creating plot-lines with twists and turns that keep the reader in suspense, without repeating herself?

"Readers are smart, and if you try to give them the same story dressed up a bit, they'll say, 'Oh I used to like her, now try so-and-so,'" Higgins-Clark said. "So never take a reader for granted."

She told CNBC that "putting a new book out" is still a thrill. "Absolutely, it's like having a baby. Is it healthy, and all the rest of it. You do the best you can on it but you wonder how it will be received."

Each year, she releases two books. "I do one in the spring around this time, and I do a co-authored one in November," Higgins-Clark said. She's written three with Alafair Burke in their "Under Suspicion" series.

"I can work seamlessly with her," Higgins Clark said, "we just finished our fourth book and there will be two more. "

So what's her method for writing a compelling mystery?

"You have to do a synopsis. You should be able to tell in a couple sentences what the book is about. Just to yourself, of course," she added.

The next step in the process is asking yourself, "who are the characters?" She said she writes a bio of each of them — "and then the names are so important. "

Another key is keeping the reader turning the page. In her sit-down with CNBC, she effortlessly set the scene of a thriller with just a few lines.

"I think that if people are anticipating, it's something like, "You're alone in the house. It's totally dark and it's locked. And then you hear footsteps on the stairs. And you reach for your cell phone and you drop it. Then the door opens…."

The rest could be coming to a bookstore near you.

On the Money airs on CNBC Saturday at 5:30 am ET, or check listings for air times in local markets.