The popularity is thanks, in part, to the much-maligned Common Core, which was introduced in 2009. 

No, really. The Common Core, a set of English language arts/literacy and mathematics goals for students at each grade level, has come under intense criticism from parents and teachers alike for its sometimes confusing implementation and odd (to grownups) ways of solving math problems. 

But there's a bright spot, and that's the Common Core's focus on nonfiction. One core goal is getting younger children to read and understand books about history, science and real life, called informational texts in "core" language. 

Years of reading fiction -- which often has more dialogue and less complicated sentence structure than nonfiction -- have left middle and high school students ill-prepared to read the texts necessary to research and write longer term papers, says Kathleen Odean, an expert on Common Core nonfiction and young adult literature. 

"Historical fiction has always been popular, but Common Core kicked it (and historical nonfiction) to the forefront," said Seira Wilson, a senior books editor at Amazon.com. "In the last year or so, it's really ramped up, with the 'Who Was?' books and 'Where Is?' series. Also, Brad Meltzer's 'I Am'