From Maboroshi's translation of the backcover:

Kanon Matsukura is a high school sophmore. Her name is based on her father's favorite classical "Kanon"... One day in school while she was walking down the hallway she hears "Kanon" being played. It was her math teacher Azume playing it on the piano awkwardly. Since then, "Kanon" keeps playing in Kanon's head. A touching story of a high school girl in love!

Also includes short stories: Happy Rain / Bang!! / Santa is Coming