Nogizaka Haruka no Himitsu (乃木坂春香の秘密, lit. "Haruka Nogizaka's Secret") is a Japanese light novel series by Yūsaku Igarashi, with illustrations by Shaa. The series originally started serialization in volume thirty of MediaWorks' now-defunct light novel magazine Dengeki hp on June 18, 2002. One more chapter was published until the first bound novel of the series was released, though more chapters were later serialized in the magazine. The first novel was released on October 10, 2004, and as of June 10, 2009, ten volumes have been published by ASCII Media Works under their Dengeki Bunko label. A manga adaptation by Yasuhiro Miyama started serialization in the seinen manga magazine Dengeki Moeoh on August 26, 2006, also published by ASCII Media Works. An anime adaptation produced by Studio Barcelona aired between July and September 2008; a second anime season started in October 2009. A visual novel for the PlayStation 2 was released in Japan in September 2008.
Nogizaka Haruka no Himitsu is set at the private high school Hakujō Academy in Japan and centers around the male protagonist Yūto Ayase and his classmate Haruka Nogizaka. While Yūto is rather ordinary, Haruka is very attractive, intelligent, and rich. Due to this, she is the school's most unattainable girl and is so idolized that her classmates give her the French nicknames "Nuit Étoile" (Night Star) and "Lumière du Clavier" (Keyboard Light). One day, Yūto's good friend Nobunaga Asakura asks him to return a book to the school library, and when he goes to return it, Yūto stumbles across Haruka and her darkest secret—she is a diehard fan of anime, manga, and the otaku culture. Yūto promises to keep her secret hidden and the two become close friends. Yūto does his best to help Haruka with her secret hobby while developing his relationship with her.