I'm not sure specifically as I haven't read the book. But it might be about some behind - the - scenes details regarding their relationship or the events surrounding them that haven't been widely known before.
Overall, Gotham has some similarities to the comics, but it also makes significant changes to characters and plotlines to make it more unique for the TV series. For example, some characters' personalities and backstories are altered.
In most cases, Gotham is canon to comics. It incorporates many elements and storylines from the comic universe, maintaining consistency and respecting the source material. However, there might be some creative interpretations or minor deviations for the sake of the TV show's narrative.
One great 'Mickey x Oswald fanfic' is 'The Adventures of Mickey and Oswald'. It's a really cute story that shows their friendship in a new and exciting way. They go on all sorts of adventures together and it really brings out their unique personalities.