“In a publishing world that maybe doesn’t have as many long-term relationships as it used to, she invested a lot of time in me before I ever earned a profit,” Mr. Green said. “I’ve never written a book without Julie. I wouldn’t know how to do it.”

Mr. Green’s rapid ascent in many ways reflects the growing dominance of young adult literature in contemporary pop culture. Last year, “The Fault in Our Stars” was inescapable, as bookstores created entire John Green sections near their cash registers, the movie’s soundtrack invaded the airwaves and people flocked to the film, which grossed $125 million in the United States. The novel, with its hardcover, paperback and movie tie-in editions, held three of the top 10 spots for the best-selling print books of 2014, according to Publishers Weekly.

In its latest financial earnings report, Penguin Random House, which operates nearly 250 imprints globally, cited “The Fault in Our Stars” as the company’s biggest hit last year, and said that “major best sellers, especially in the field of children’s books” had helped produce a 25 percent increase in revenue in 2014.

This year, in anticipation of the forthcoming film adaptation of Mr. Green’s 2008 novel, “Paper Towns,” Dutton is printing 1.5 million copies of a movie tie-in edition. To put that figure in perspective, the year’s most highly anticipated new adult fiction title, Harper Lee’s second novel, “Go Set a Watchman,” will have a print run of two million copies.

Teenage-oriented publishing took off in the 1960s and 1970s with best-selling books by Judy Blume, S. E. Hinton and Lois Duncan. The children’s category began to grow and evolve rapidly in the late 1990s. The breakout success of J. K. Rowling’s Harry Potter books, which attracted hordes of adult readers and now have sold more than 450 million copies worldwide, paved the way for other crossover hits. They arrived one after another — “Twilight,” “The Mortal Instruments,” “The Hunger Games,” “Divergent” — and started a stream of blockbuster Hollywood franchises, which in turn brought older readers to the books.

The lines dividing adult and children’s literature have grown increasingly blurry, as young adult authors experiment with dark, challenging and sophisticated story lines and narrative structures. Adults aged 18 to 44 made up 65 percent of young adult book buyers in 2014, according to a recent Nielsen Books & Consumer survey, and men accounted for 44 percent of young adult book buyers in 2014, up from 31 percent in 2012. And 65 percent of adults buying young adult books reported that they were purchasing the books for themselves rather than for children.

As the appeal of young adult literature expands beyond its intended demographic, blistering debates have broken out on the web over whether it’s appropriate for adults to read children’s books. Some see the rapid rise of young adult fiction sales as a potential threat to American literary culture. Not only are a lot of grown-ups reading children’s books, it seems they’re reading them instead of adult books.