Side story from Gosick.

Gosick takes place in 1924 in a small, made-up European country of Sauville. The story centers on Kazuya Kujo, the third son of a Japanese Imperial soldier, who is a transfer student to St. Marguerite Academy, where urban legends and horror stories are all the rage. There he meets Victorique, a mysterious yet beautiful and brilliant girl who never comes to class and spends her days reading the entire content of the library or solving mysteries that even detectives can't solve. The series mostly focuses on Kazuya and Victorique getting involved in different mystery cases and their struggle to solve them, at the same time forming important bonds with different people.

This short manga is based on the novel; Sugar Candy Bullets Can’t Pierce Anything.
The plot follows two girls and their summer-time adventures in a rural area.
Nagisa Yamada is a bored teenager who finds everyday life extremely unappealing - thus she plans on joining a Military School as soon as possible.
A strange new student named Umino Mokuzu (who claims to be a mermaid) transfers into Nagisa's school and, much to our heroine's bewilderment, soon begins to follow her around incessantly.
Initially, an annoyed and discomfited Nagisa strives to avoid any encounters with Mokuzu. But she soon begins to comprehend the pain and sadness concealed beneath this strange girl's cheerful and carefree demeanour...

Yamanouchi Kouya is a famous author's daughter, who is a shy young girl just starting middle school. She is named class representative along with Kannatsuki Yuuya, a boy she met on her way to school who frightens her a little bit. They are trying to work together, but he always seems mad at her and she is painfully shy. The thing is, Yuuya knows something that Kouya doesn't know yet—that her famous father is going to marry his mother!
(Source: Dazzling Scans)