"'Woo hoo,' Mercer yelled, with his ears flapping in the wind…" Shea reads from the book.
Shea travels with her mother, who helps her write the books, and a service dog, CBS News correspondent Richard Schlesinger reports.
The fact that she travels at all is remarkable, because she suffers from Spinal Muscular Atrophy - a paralyzing disease.
"I was born with this disease. And, you know, I just grew up with it, grew up accepting it, and I moved on," she explains.
Her first book began as a few entries in her diary, discovered by her mother - sketches of Mercer ice skating … by a little girl who never would.
"And so … you think that Mercer is really sort of her, in her imagination?" Schlesinger asks.
"Yeah, I do," Megan Megale, Shea's mother, said. "And I think he's an outlet for her, too."
But Mercer is not just an imaginary friend. He's very real.
And Schlesinger met Mercer, the real dog.
He's a four-legged metaphor for a life lived in a little girl's mind. He goes everywhere with her, helping her with everyday chores and inspiring her fantasies.
"Why did you choose Mercer to be the star of your books?" Schlesinger asks.
"Well, I thought he was the star of my life. So I thought why not the book? He's really amazing and he can, he's my best buddy and he deserved it I thought," Shea says.
It's her escape from the reality of her disease, which doctors say will probably kill her by her late teens.
At 12 years old, she's already too weak to move very much.
But she is moving others.
Her book has been picked up by FAO Schwartz and Amazon. And more books are in the planning stage. Mercer will go swimming and sledding and all sorts of other things … things that Shea knows she'll never do.
But she says it doesn't make her sad.
"No, it doesn't 'cause, you know ... I'm sure there's stuff you can't do, am I right?" Shea says.
"It gives her a whole new identity of, you know what, instead of this is what it is and I've gotta find a way to live my life to showing people this is how you live life," Megan Megale says.
But Mercer makes all things possible for Shea. He's her muse, her hero, a central character in both her lives … real and imaginary.
To visit Mercer's Web site, click here.