The blazingly popular Who Is?/Who Was? series of biographical books -- which can be found in many a child's backpack, bed, desk, or minivan backseat -- is celebrating its 15th anniversary this year and, surprisingly, almost never became the sensation it is today.

"For several years the series just limped along," says series creator Jane O'Connor, also vice president and editor at large at publisher Grosset & Dunlap. "It stayed alive primarily because book fairs would buy up copies in bulk, and schools and libraries bought them, but there was virtually no bookstore business for the series, and we really needed that."

But fortune changed for the now 100+-book-strong series when Grosset & Dunlap brought in a new president, publisher, and head of sales, all of whom believed in the series' appeal. O'Connor says the trio sparked its resurgence and rapid-fire expansion, which continues unabated today.

"I was so worried that the series would die," she says. "I was so invested in this. I was such a reader of biographies, it was thrilling for me to see this really reach a wide audience."

The series, geared for 8- to 12-year-old readers, offers 100+-page, illustrated, single-subject biographies of a range of public figures from history, government, entertainment, pop culture, sports, and more. It began with four titles, Who Was Annie Oakley? Who Was Ben Franklin?, Who Was Albert Einstein?, and Who Was Sacagawea?, and was inspired by O'Connor's childhood love of the biographical series, Childhood of Famous Americans.

"I had been an obsessive reader of biographies when I was a child," she recalls. "I started to think there wasn't really a series [today] that was for kids who were beyond easy-to-read books that were 32 pages, and yet weren't ready for a really dense, text-heavy 150- to 200-page book. I liked the idea of having something very heavily illustrated and at a third-grade reading level, so kids who were in second, third, or fourth grade could have a book they could use for a book report. It would be over 100 pages, and yet it would actually not be all that much reading."

O'Connor returned to her favorite series for inspiration for the Who Is/Was iconic covers: "Childhood of Famous Americans had only black silhouette illustrations. They were always orange covers with no illustration on the front. You knew the books immediately."

O'Connor says she liked the idea of a signature look, and was inspired by the then New York Times Sunday Book Review, which when covering a new release or biography, would feature a large bobble head caricature of the author or biography subject. O'Connor turned to artist Nancy Harrison to draw the large-headed caricature covers, which she still does today.

"I thought something like that would stamp the series as a series and appeal to kids," O'Connor says. "They would know the series instantly; they often might recognize who the person was. And when you stack them all together, they look like trading cards."

The trading card comparison is an apt one, as the series' young readers tend to be completists. They often can't read just one, moving from one title to another, with no end in sight.

"The series just keeps expanding, and yet it's not like the core books have lost readership," she notes. "They are really selling well continuously."

Despite over a dozen new releases every year, O'Connor says years-old titles, such as 2002's Who Was Harriet Tubman?, continue to be best-sellers ("She is one of the most popular," O'Connor notes). Titles can also jump in popularity thanks to current events, such as the news that Tubman will replace Andrew Jackson on the $20 bill. O'Connor says the upcoming Who Was Andrew Jackson? was being edited at the time the announcement was made, which led to a new ending for the book: Jackson losing his spot on the $20 bill.

Some of the most popular titles in the 100-book series include Walt Disney, Dr. Seuss, Bill Gates, Marvel Comics icon Stan Lee, and author Jeff Kinney. While the covers always sports Harrison's signature style, the books are written by a series of authors. O'Connor's personal favorite? Who Is Jane Goodall? --"I'm just crazy about her," she confesses.

Choosing the subjects

With a millennia worth of potential topics, the series turns to its core audience to determine which subjects to write next.

"We try to go to schools a few times a year and ask kids who they'd like to see," O'Connor says. "A few years ago, I was in a classroom and loads of the kids asked for Michael Jackson. I had really shied away from doing him. There was a librarian and a teacher in the room, I looked at them, and they said, 'No, you should do him, he's a cultural icon.'"

And, she adds: "Occasionally I've been told, 'No, you can't do that book.' When Osama bin Laden was killed, I wanted to do Who Was Osama bin Laden? I was the only person who thought that was a good idea."

The series stakeholders also talk to bookstores, who relay reader requests, and they gear new releases seasonally. For example, Who Is Hillary Clinton? was published in August, Who Is Michael Jackson? debuted before Black History Month, and Who Is Andrew Jackson? will hit shelves before the next President's Day.

The book whose popularity surprised O'Connor the most is Who Was Anne Frank? She was unsure if their target readers would be familiar with Frank and her compelling story.

"I think of the [series'] sweet spot as between second and fifth grade, and I wondered if Anne Frank was someone who kids come to when they read the diary when they're older. But that book does very, very well," O'Connor notes.

As the series approached its 100th title, the publisher held a contest in which kids could vote for the book's subject. The event resulted in 2015's Who Was Steve Irwin?, another surprise for O'Connor, who was curious if children would be familiar with Australian naturalist, given he died a decade ago.

The series branches out

Three years ago, the series expanded, moving from people (and Seabiscuit) to places and events with the What Was?/Where Is? titles. These books can delve into heavy topics for young minds, like What Was The Holocaust?, the Great Depression, D-Day, the Hindenburg, and more.

"What is tough for authors to understand is what that reader is going to come in knowing and not knowing, and that they're going to not know practically everything," O'Connor says. "You can't assume that they have any background coming into a subject. With older kids, you can assume they know about the Revolutionary War or that the U.S. was colonies before. With kids [in the What Was? age range], you really can't, you have to give them all the background information. Kids coming to these books have hardly any historical context. If you're writing about Anne Frank, you have to have the rise of Hitler, and what came before she ended up in the attic."

She adds that series authors are very conscious of the fact they may be a child's first introduction to a famous figure or event: "My husband wrote What Were The Twin Towers?. We talked a lot about it as he was writing it. We wanted to give a sense of what happened, but not be overwhelmingly terrifying for kids. We're very conscious of the age group."

There are, however, plenty of lighter What Was? topics, such as What Are The Summer Olympics?, What Is The Super Bowl?, and more. Subjects in the Where Is? series include The White House, The Great Wall, The Parthenon, the Amazon, and more.

Releases coming kids' way this fall include:

Where Is the Great Barrier Reef?

Where Is Stonehenge?

Who Is Bruce Springsteen?

Who Is Stevie Wonder?

Who Were The Three Stooges?

What Was the Great Chicago Fire?

What Was the San Francisco Earthquake?

Who Was Jacqueline Kennedy?

Who Was Charlie Chaplin?

"I was worried we were doing too many," she says, "but so far, there seems to be an endless appetite."