Blue Fingers: A Ninja's Tale, Cheryl A. Whitesel
Fantasy. Blue Fingers is about a young boy named Koji who gets captured by a clan of ninjas, and is brought to their mountain base to be trained to be a ninja- or be killed. Early in the book, Koji looks like he's leaning more toward the second option, as he keeps on trying to escape back to his home on the farm. Eventually he becomes an accomplished ninja and learns how to become "a blade of grass" (you'll find out what that means after you read the book). 9+ for some violence.