John Cena’s newest gig? Children’s book author

John Cena noticed something weird during his Doctor of Thuganomics days in the WWE.

He studied the crowds coming to see the events and noticed a lot of children in the mix.

"It was sophisticated market research. Me poking my head out of the building and looking at the people filing into the building. I could see more families coming," said Cena, who, around 2005, started a transformation that, in a way, has led him to the Central Library of the Free Library of Philadelphia, where Friday, Oct. 12, he'll sign copies of his new children's book Elbow Grease.

Cena took that "sophisticated research" and used it to his advantage.

"At the time, as the Doctor of Thuganomics I was exploring my hip-hop persona, and that was fun, but he was not exactly the most palatable guy for a family audience, so I asked if I could evolve into a different kind of image," Cena recalled.

Again, he took his cues form the crowd.

"I looked at what they were wearing, at the signs they were carrying, and paid attention to what they cheered for," he said. Cena started sporting a crew cut and wearing fatigues, the mantle of patriotism, a move that was reflected in his offscreen persona; in 2006 he made his first movie as a lead, The Marine (he's since found success in more comedic roles, like a memorable cameo in Trainwreck and a top-billed slot in Blockers). He also started winning, and became the good guy face of the WWE.

With great success, for Cena, came great responsibility. He started working with the Make-A-Wish foundation, again using the special connection he'd developed with children. He's done over 500 "wishes" for the foundation, the most of any celebrity. What he learned from working with those children he put into his first book, written for young readers.

"Never give up. That's absolutely the thing. I've met so many families that are facing circumstances that are just hard to fathom, and it's amazing to see the children's attitude," said Cena, who wanted to capture that spirit in Elbow Grease.

It's the illustrated story of a little truck, the smallest vehicle at the demolition derby, and the smallest of five brothers (Cena has four, not coincidentally), trying to learn to survive and to thrive. Elbow Grease is the name of the truck, and also the attribute that Cena wanted to celebrate in the story – the extra amount of hard work it takes to get the job done.

"The other trucks have measurable attributes. They are big, or strong, or fast, but I wanted Elbow Grease to have something intangible. I call it gumption," Cena said. "I like that word, because it connotes more than just hard work. It also means being resourceful, being creative. Also the ability to be steadfast – to fail, but to keep going."

Cena said gumption accounts for "100 percent" of his success as a performer, and sees the same concept circulating elsewhere in the culture, where it's also celebrated as "grit."

"You see [Penn psychologist and MacArthur Genius Grant winner] Angela Duckworth and others putting forth the message, and I think it's great."

Books meant a lot to Cena as a tyke in pre-internet exurban Massachusetts ("a half acre from anyone"), where he liked to hunker down with the "Busytown" stories by Richard Scarry.

"Somebody came to me with the idea of doing a book, and an autobiography was suggested, but that didn't appeal to me," said Cena, who instead recalled how absorbed he became in Scarry's make-believe world. Writing something for younger readers seemed a natural fit with biggest fan base.

Will he write another?

"I never thought there would be a first book so this is all great. I love the book, and I put all of my heart and soul into it. But it's up to the consumer," said Cena, who's traded thuganomics for plain old economics.