Parasyte 1 by Hitsoshi Iwaaki. In this fantastic horror manga reprint, an alien parasyte invades earth, causing the humans it possesses to cannibalize their fellow humans. A young boy accidentally halts the creature's growth before it reaches his brain, turning his hand into an otherworldly creature with a mind of its own. Though it's motives are always described as enlightened self-interest, the boy and creature develop a strangely touching symbiosis underscored by the splatter horror of the main plot. Continued in a second volume, it's a story fans of j-horror, Uzumaki, and teen slashers should appreciate.