WASHINGTON — Shea Megale didn’t understand just what she had achieved until the boxes of books started arriving at her Centreville, Va., home. Inside were thousands of copies of Marvelous Mercer.

At the age of 12, Shea had become a published author. She suddenly felt “very accomplished.”

Her book, the first in a planned series of six, is about the fictional adventures of her very real dog, Mercer.

In the first story, released last month, Mercer goes ice-skating — something Shea can’t do because she has Spinal Muscular Atrophy.

SMA is a genetic disorder that severely weakens the voluntary muscles (the ones you control). It usually leaves a person unable to walk and, in some cases, unable to stand or sit. There is no known cure.

Shea has had SMA since birth. Three years ago she received Mercer from Canine Companions for Independence, a group that trains dogs to help people.

Mercer, a 5-year-old black Labrador/golden retriever mix, helps Shea in many ways — including picking up things she drops, turning on lights and opening doors.

Shea calls Mercer “my best friend” and wants to raise money so that Canine Companions can train more assistance dogs. Thirty percent of the money her books make will go to that group and to SMA research.

Marvelous Mercer began as a series of diary entries. Shea’s mom, Megan, read them and thought they would make a good book.

Mom took on the business end of publishing, leaving Shea to concentrate on her writing. Her uncle, Curt Wagner, painted the pictures.

Even with her family’s help, Shea said, she spent less time writing the book than she did attending meetings about it. Getting together with editor Catherine Conley, her uncle and a publicist took lots of time.

Although her editor was nice, Shea said, being edited was “harsh” because “your stuff is changed.”

“You don’t know what you are getting into,” she said of the publishing process. “It is very difficult.”

Shea hopes that her books — the second one is scheduled to be out in January — educate people about the role of assistance dogs and what it’s like to be a kid in a wheelchair. And she’s pleased that money from the sales will help others.

“It is such a positive book,” she said. “I hope it benefits everyone and anyone.”

Check out Shea’s Web site: www.marvelousmercer.com.