The Last Book in the Universe by Rodman Philbrick
This book tells a tale of ancient myths and legends, ghostly phenomena, black and white magic and the blurring of god and evil. But this is not fantasy. There is plenty of magic in the real world if you know where to look. In a land dominated by vicious gangs and mind probe entertainment, Spaz is alone. His foster sister is the only good thing in his life and she is dying. Determined to save her, Spaz bravely sets out into dangerous and forbidden territory, accompanied only by an old man, with his philosophies and memories of what the world once was.