One popular story could be 'A Day in the Park'. It's about Sasaki and Miyano having a sweet day out, sharing ice creams and having deep conversations. There are also those that focus on their school days like 'School Festival Shenanigans' which details their experiences during the school festival, with all the cute interactions and misunderstandings.
You can start by looking on fanfiction platforms like Archive of Our Own. Just type in 'sasaki and miyano' in the search bar and it'll show you a whole list of fanfictions related to them. You can also check out some anime - focused forums where fans might recommend their favorite stories.
They have a complex and evolving relationship. At first, they are just acquaintances, but as the story progresses, they grow closer and develop deeper feelings for each other.
The light novel is unique in its character development. Sasaki is initially seen as a bit of a playboy but as the story progresses, his deeper, more sincere side is revealed. Miyano, on the other hand, is shy and reserved about his own feelings, and his growth throughout the story is engaging. It also has a unique art style in its illustrations which add to the overall charm of the story.