Shigeru Mizuki's Kitaro is one of the most famous and influential series in Japanese manga history. Why is it so important? Because, through works like Kitaro, Shigeru Mizuki is responsible for popularizing Japanese folklore, and to some extent, creating it as well. Kitaro is a compilation of short episodic stories written between 1967 and 1969.
It features the hero Kitaro, who along with his father (an anthropomorphized eyeball), is the last member of the ghost tribe. He challenges legendary monsters (or yokai) and does good deeds to fight for peace between monsters and humans.
As fun as the stories are (and they are really fun!), the most interesting bit of this manga to me is the visual "monster" glossary at the end of the book which explains all of the mysterious and bizarre Japanese monsters Mizuki draws. Fascinating!