Sound! Euphonium: True Stories from the Kitauji High School Concert Band

Sound! Euphonium: True Stories from the Kitauji High School Concert Band (響け！ ユーフォニアム 北宇治高校吹奏楽部のホントの話) is a spin-off novel from the main series. It was published by in Japan on April 5, 2018.

Synopsis
The book contains a series of short stories that take place over the course of the characters' lives. Each chapter is a different story, and is told from different perspectives.

Chapter 1
The chapter is told from the perspective of Nozomi Kasaki. It takes places the day of the third-years' graduation. Nozomi bumps into Natsuki Nakagawa on the way to school, and later they find Yuuko Yoshikawa crying with Mizore Yoroizuka. Natsuki and Yuuko engage in a fight, and Mizore cryptically explains to Nozomi what had occurred between her and Yuuko.

Chapter 2
Kumiko Oumae, Reina Kousaka, and Hazuki Katou meet up to do homework at Sapphire Kawashima's house per Hazuki's suggestion. It takes place soon before summer vacation. Sapphire is revealed to have been studying very hard to become a reliable source of help for her kouhais in place of Asuka Tanaka, and praises the importance of studying. They also discuss their kouhais' behaviours regarding school and relationships.

Chapter 3
The chapter begins with a letter from Kaori Nakaseko to Asuka Tanaka the day before their graduation. Kaori notes that one of the only things she regrets was not responding to Asuka's question. The chapter then transitions to partway through Asuka and Kaori's first year. The two discuss the problems in the band, and predict the possibility of the band schism. Later, Kaori confesses her love for Asuka, though not wholly clear whether platonic or romantic. The chapter transitions back to Kaori's letter, where she continues to profess her feelings for Asuka.

Chapter 4
The chapter is told from the perspective of Aoi Saitou. It takes place during her university entrance ceremony. She encounters a girl named Kyouko Itou. The two discuss their common interests, music being one of them, and Aoi decides she wants to play tenor saxophone again. At the concert band's first-year initiation, Aoi bumps into Haruka Ogasawara, who happened to be visiting as her university did not have a concert band.

Chapter 5
The chapter is told from the perspective of Kanade Hisaishi. She goes to Ririka Kenzaki's on their day off, where the two make œufs à la neige (floating island dessert). The two banter with each other while preparing the dish.

Chapter 6
The chapter is told from Serina Hiiragi's perspective. It takes place in her class while people are discussing relationships and crushes. Kumiko Oumae and Shuuichi Tsukamoto's relationship is brought up, and they begin talking about the school's concert band. Sapphire Kawashima is informed that Serina has a question about music, and the next day Serina asks her about Rikka's band. Serina later reveals she was invited to attend the competition by Azusa Sasaki.

Chapter 7
The chapter is told from Haruka Ogasawara's perspective. She along with Kaori Nakaseko and Asuka Tanaka are at a ski lodge for their graduation trip. The three go to the onsen, where they discuss their own futures and the future of Kitauji's concert band.

Chapter 8
The chapter is told from Satomi Niiyama's perspective. It takes place while she is still in music school. She is accompanied by her senpai, Chihiro. The two get on the topic of relationships. Chihiro is revealed to have been dating Noboru Taki for a year, while Satomi is yet to enter a relationship. Chihiro mentions she wonders what would happen if something suddenly happened to her, specifically how Taki would respond.

Chapter 9
The chapter takes place in the midst of their lunchtime during holiday practice. Hazuki Katou comments on Satsuki Suzuki's twin-tails, and Satsuki asks Hazuki to tie her hair up the same way. Ririka Kenzaki and Kanade Hisaishi quickly caught on and tied their hair up too. Soon enough, their whole group has their hair tied up in twin-tails.

Chapter 10
The chapter is told from Nozomi Kasaki's perspective. It begins with her picking up a cake for her family's Christmas celebration. On the way out of the store, Nozomi bumps into Natsuki Nakagawa, and the two decide to get tea. Inside the cafe, the two discuss what they plan to do in university. Natsuki states that she will be done with concert band, and would rather play in a light music group, but Nozomi tries to convince her that picking up the euphonium again later in life wouldn't be a bad thing.

Chapter 11
The chapter is told from Riko Nagase's perspective. She and Takuya Gotou are waiting for the bullet train to arrive for him to leave for Tokyo, where he'd be moving to study. Riko becomes overwhelmed with emotion over his departure, and is overcome with memories of their lives in high school.

Chapter 12
The chapter is told from Kumiko Oumae's perspective. It takes place during October, once she has inherited the role of band president from Yuuko Yoshikawa. Kumiko announces to the band that they will be participating in the Ensembles Contest in December, where they would only be sending one small group. After a discussion with Taki-sensei, they decided that the selected group should be determined by a ballot, voted upon by students. Reina formed a group for an octet of Ayabu ~Ibuki~ with Kumiko, Shuuichi Tsukamoto, Hazuki Katou, Yume Kohinata, Michiyo Morimoto, Junna Inoue, and Tsubame Kamaya.

Chapter 13
The chapter begins with a letter from Yuuko Yoshikawa to Natsuki Nakagawa shortly before their graduation, since Kaori Nakaseko did the same. It then shifts to Yuuko's daily walk to school, where Mizore Yoroizuka sneaks up on her. Mizore thanks Yuuko for helping her and being her friend. Yuuko is overcome with emotion and bursts into tears. Natsuki and Nozomi Kasaki approach them, and Yuuko quickly starts bickering with Natsuki while Nozomi walks off with Mizore. The chapter transitions back to Yuuko's letter. She thanks Natsuki for keeping her under control and always being by her side during all her years.