i've read kokoro too! its a great book written in early 20th century. very poetic and beautiful. yah.. and kinda sad and lonely.
other famous classic writers includes yasunari kawabata who won a noble prize for literature and junichiro tanizaki who wrote more kinky stories. if you are into classical writers, can find out more about them.
but if you are looking for something more contemporary, try ryu murakami's 'coin locker babies'. powerful and wicked. violent, but i think it reflects also partly the psychic of japanese city people and how the negative effects of modern society, or at least what murakami thinks. that's another depressing story you have there.