Ore no Imōto ga Konna ni Kawaii Wake ga Nai - the light novel series published by Ascii Media works. Also known as "Oreimo" and often abbreviated to "Ore no Imouto."
ANN has a nice short description of the story:-The story follows the daily life of an ordinary high school boy named Kyōsuke Kōsaka. Kyōsuke's younger sister Kirino is a pretty fashion model — but also a secret otaku of adult games. Kyōsuke is the only person to whom Kirino reveals her hidden obsession and other issues.
Been reading the novel for a while now which brought back memories of how we used to read light novels like this when studying Japanese back in the UK. We read a load of Hoshi Shinichi's novels which provided us with not only an interesting read but also learned a load of new words and kanji too. If you are learning Japanese then I do recommend trying to read light novels because it helps stimulate your imagination through a foreign language that will help towards your study.