Squirlish: The Girl in the Tree by Ellen Potter
Cordelia is a girl who lives in a tree in Central Park. Found as a baby and raised by an adoring squirrel named Shakespeare, Cordelia acts just like any other young squirrel, leaping across treetops, chasing her squirrel friends, and sleeping in her treehouse. Still, she wonders what it would be like to have a human friend, and when she stumbles into a gymnastics class, it seems like she might have her chance.