This holiday season, don't be surprised to see more jostling for presents under the tree. Not just kids, but adults competing for the same gifts.
Nielsen says 80 percent of the market for young adult books is now over the age of 18.
The overall category of kids' books is the fastest growing in publishing, with sales up 24 percent since 2010.
When R.J. Palacio sat down to write a children's book, she tried to put herself in the mind of a 10-year-old.
But the story of a fifth-grader struggling with a facial disfigurement struck a nerve everywhere.
"We've all been outsiders. We all remember what it's like to be either the new kid or the different kid," said Palacio.
"Wonder," a simple story with short chapters, has sold more than a million copies in 18 months and been on The New York Times bestseller list for 53 weeks – propelled by adults.
Palacio says she was "completely surprised" by the book's success. "There’s no way I would have predicted this would have happened."
Kids’ books, long given less attention by big publishers, are suddenly bigger than ever.
"Now we are one of the most profitable departments of Random House," said Erin Clarke, young readers editor at the publisher's Knopf division. "You know, people pay a lot of attention to us now."
The titles may not sound sophisticated, but the salse are. The "Dork Diaries" series sold 13 million copies and “Diary of a Wimpy Kid” 115 million.
And the biggest names in publishing are now penning young adult books, like James Patterson and David Baldacci.
"They're huge," said Clarke. "They’re sitting on The New York Times best-seller list and they are just as big as their adult titles."
The phenomenon has its roots in part with the "Harry Potter" series, which was embraced by all ages, selling 450 million copies and leading to a blockbuster movie franchise.
"It's a great happy surprise, one that everyone is milking," said book agent Alyssa Eisner Henkin. "But I don't think it's one we necessarily would have predicted and we don't know how long it will last."
Lion's Gate, which made "The Hunger Games," has shown interest in turning "Wonder" into a movie. A screenplay is in the works.