Although Diane was hired as a ghostwriter and it's meant to be a memoir for BoJack (the latter even casually mentions to her family in Live Fast, Diane Nguyen that Diane's name won't be on the book).
One Trick Pony is constrained to third-person through Diane's perspective rather than BoJack's. Her name is prominently displayed on the cover—making the book a biography.
BoJack initially isn't a fan of the book, because he is embarrassed that he came off as a terrible person, however, he has grown more accepting of it over the course of the series.
In Later, the book won a Golden Globe for Best Comedy or Musical, despite being neither a comedy or a musical.