The Green Ninja is a Ninjago: Masters of Spinjitzu book that was released in 2013. It takes place within the plot of Season 2: Legacy of the Green Ninja, during the events of the episode "Child's Play."
Masters of Spinjitzu: still a force to save the world!
Lord Garmadon is up to his old tricks. He's using his Mega Weapon to turn back the clock and bring the fossil of an ancient monster back to life. That's when Cole, Jay, Kai, and Zane interrupt him . . . and get turned into kids again! Now the mini-ninja must do whatever it takes to grow up . . . and stop the monster from destroying Ninjago City!