After author David Baldacci agreed to write a book in the popular "The 39 Clues" series for kids, he needed some basic information before he could begin creating the story.
Mr. Baldacci was used to writing for adults; he has written 25 best-selling murder- and mayhem-filled thrillers that have sold more than 110 million copies worldwide. But "The 39 Clues" series, published by Scholastic, is aimed at readers ages 8-12.
"So I met with the Scholastic folks and said, 'Let's set forth some parameters,' " Mr. Baldacci said in a recent telephone interview. "I asked them, 'Can I kill people?' They said, 'Yes, people have died in these books.'
"Then I asked, 'Is there anybody I can't kill?' Their answer was, 'Well, the brother and sister, Amy and Dan Cahill, are the mainstay of the series, and we'd like to see them survive.'
"So I said, 'Can we maim them?' By this time, I was just playing with [the Scholastic editors]. They looked a bit stunned and as if they wondered if I should be writing this book, before they realized what was going on."
Mr. Baldacci was tapped by Scholastic to write the final book, "Day of Doom" ($12.99), in a six-book series, "Cahills vs. Vespers." That series is a sequel to "The 39 Clues," an 11-book series that has sold 15 million copies since it was launched in 2008. Various authors have contributed to at least one of the series.
In "Day of Doom," Mr. Baldacci offers younger readers a taste of his talent for writing can't-put-'em-down thrillers. Here, Amy and Dan Cahill learn that their efforts to ransom seven other members of their family from the evil Vespers clan haven't worked.
Instead, it appears that Vesper One, the leader of the Vespers, will stop at nothing to take over the world, harming millions of people, including the hostages, along the way. Only Amy and Dan can stop Vesper One, but even they aren't sure that it can be done before the Vespers set off the Doomsday machine.
Young readers will find themselves eagerly turning the pages of "Day of Doom" to find out what happens next. Mr. Baldacci references previous events while putting his own spin on the plotline.
While his plot takes center stage, Mr. Baldacci doesn't neglect his characters, especially spotlighting the tight, if not always perfect, sibling bond between Amy and Dan.
The print edition of "Day of Doom" is just one part of the story presented to young readers. "The 39 Clues" series was a pioneering force in the new trend of "multiplatform" books, which involves adding other elements to the print book.
In the case of "The 39 Clues" and "Cahills vs. Vespers" series, readers also can play a connected online game, comment via a message board and enjoy collectible cards included in a special compartment inside the front cover of the books.
"We've known for a long time that books have lots of other competition for kids' attention," Rachel Griffiths, the Scholastic executive editor who worked on "The 39 Clues" from its inception, said in a separate telephone interview.
"So we thought, 'If we can't beat the competition, let's join it,' and created this multiplatform series."
For "Day of Doom," Mr. Baldacci asked Scholastic editors if they'd also be interested in creating a webcast -- a virtual field trip -- featuring a behind-the-scenes look at the Smithsonian Institution's National Museum of American History.
One of the museum's artifacts, the compass used by explorers Meriwether Lewis and William Clark, is featured in "Day of Doom." Mr. Baldacci, a history buff, thought that the webcast could highlight the compass along with other items that are part of the museum's collection, including President Abraham Lincoln's pocket watch and the flag that inspired Francis Scott Key to write the lyrics to the "The Star-Spangled Banner."
The webcast first aired on March 5, but can be accessed, for free, at www.scholastic.com/decodinghistory.
"History and books, what could be better?" Mr. Baldacci said. "Each entertains and educates.
And 'The 39 Clues' series and the Smithsonian do both, splendidly."
In a series for kids, Mr. Baldacci said he obviously "couldn't have all the elements" that he does in his adult series. But he still wanted to offer young readers "lots of twists and turns, surprises and danger."
While "Day of Doom" is the final book in the "Cahills vs. Vespers" spinoff of "The 39 Clues" franchise, Ms. Griffiths said the series isn't about to end.
"We have a four-book series launching in October," Ms. Griffiths said. "As long as kids are interested in reading these books, we're going to continue to publish them."
Meanwhile, Mr. Baldacci is thinking of writing more books for kids.
"I've definitely had a fire lit under me," he said. "It was a lot of fun writing 'Day of Doom' and great exercise for another part of my imagination."
Karen MacPherson, the children's/teen librarian at the Takoma Park, Md., Library, can be reached at Kam.Macpherson@gmail.com.