If you’re looking for Korean graded series written completely in Korean, there is a book series called “외국인을 위한 한국어 읽기 100” for intermediate learners, consists of 100 books.
I have only read one book before but I know books of this series are short enough to finish in one sitting, grammars used are very simple, but I still can learn many words from them. So I read one yesterday because I wanted a sense of achievement by finishing a Korean book.
I picked book no. 49 운수 좋은 날 (One Lucky Day), a short story originally written by 현진건. I bought my e-book from Google Play Books. Somebody (a Korean) on Lang-8 recommended the story to me a long time ago. I suppose it’s a story every Korean has to read at least once at school.
The story in original form can be read here: http://ko.wikisource.org/wiki/%EC%9A%B4%EC%88%98_%EC%A2%8B%EC%9D%80_%EB%82%A0. As you can see, it’s very difficult because it’s a classic literary work. But the re-written version in the book for foreigners is easy enough for early intermediate learners (see preview pages here: https://play.google.com/store/books/details?id=tCdIAwAAQBAJ.).
The story is not a happy story, about one unlucky day of a rickshaw puller living in poverty in old time Korea. ###