The Dissociation of Haruhi Suzumiya (light novel)

As the second year of high school begins, Haruhi's cynical and sarcastic confidant Kyon, the only "normal" member of the SOS Brigade, finds himself meeting Sasaki, an old friend from his days in junior high. Haruhi and Sasaki's shared friendship with Kyon and, possibly, shared abilities, causes tension between the two and Kyon knows Haurhi's dissatisfaction can have dire consequences! Kyon and Sasaki's chance meeting could affect his future, the future of the SOS Brigade, and the world forever.
This ninth volume of the Haruhi Suzumiya series is the first to use two alternate stories for the same plot with different endings. The story continues in the next novel, The Surprise of Haruhi Suzumiya.