Dragon Ball Z Volume 2 by Akira Toriyama

Goku is dead--but his journey is just beginning. Now he must travel through the afterlife along the million-kilometer Serpent Road to find Kaio-sama, the Lord of Worlds, who will teach him martial arts techniques so powerful they're reserved for the gods! And he'd better hurry, too, because Earth has only nine months until the evil Saiyans, Vegeta and Nappa, will arive to wipe the planet clean of life. While the archdemon Piccolo trains Goku's son Gohan to reach his full power potential, the alien spaceships rocket ever closer...bearing a power so awesome that even Earth's six strongest heroes may not be a match for it!!!