Haikei, Arishihi ni Saku Hanatachi e

Kasumi, a serious girl who has just started at an all-girls school, discovers a letter tucked into an old book she found in the library. The letter's writer yearns to have a special relationship with another girl, like how it was described in the book… A romance about the the old-school "S" kind of girls-love.
(Source: Kadokawa, translated)