I wouldn’t quite call this exposition as much as it is the backstory for Rosalina, but you don’t know the story’s protagonist is actually her until later. The player is also not required to read the book to understand the world of SMG, since all of that is communicated as the game progresses.
Still, it’s quite a well-written, standalone story, outside of video game dynamics. It addresses child (anthropomorphic star) abandonment and parental death without being stark or preachy about it; it’s essentially a children’s book, after all. To wit, the ending scenes of the game involves the self-sacrifice of the (child-like) stars at the death of the universe, and the subsequent reincarnation of everything. Again, this is handled without the grim seriousness you would normally see with apocalypse storylines. And the secret ending involves another abandoned child (anthropomorphic star), adding to the cyclical nature of SMG universe’s…universe. When did Super Mario games get so deep?