Well, without more information about this 'sayonara novel', it's hard to say precisely. It could be a coming - of - age story where the main character has to say goodbye to their childhood innocence or a group of friends. It might also be a love story where two lovers part ways and the novel explores the pain and growth that follows this separation.
Sayonara fanfic could refer to fan - created fictional works that are related to the concept of 'sayonara' which means goodbye in Japanese. It might be fanfictions that deal with themes of parting, endings, or characters saying goodbye in a particular fictional universe.
I'm not sure of specific'sayonara fanfic' examples off - hand. But it could be like a Harry Potter fanfic where a character decides to leave Hogwarts for good and there are a lot of emotional goodbyes with friends and professors. It would be a story centered around that parting moment.
I'm not entirely sure as I haven't read it in detail. But generally, it might be about piano sonatas and perhaps some kind of emotional story related to music, like a pianist's journey, the beauty and power of piano music, and relationships that are intertwined with the creation or performance of piano sonatas.