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.
Picked up this Kirino Kousaka towel for Karin to rub herself down after a shower.
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.
Kirino is one of my fave 2D girls at the mo. Shes such a cutie!