Rosalina's Story, while never explicitly stated, is the story of how Rosalina made it into outer space and how she became the "mother" of the Lumas prior to the events of Super Mario Galaxy. In the Comet Observatory's Library, Rosalina sometimes reads from the storybook to the Lumas traveling with her. Mario/Luigi can come in to listen to the story. In all, the storybook contains nine chapters.
Yoshiaki Koizumi, director of Super Mario Galaxy, was the writer of the storybook. He wrote the script late at night when no one was around so no one would know what was in it. The next day, he told Shigeru Miyamoto about the story and presented it to him. Miyamoto was surprised that Yoshiaki wrote the script late at night with nobody around. Yoshiaki Koizumi, in Nintendo Power, mentions that he would enjoy seeing Rosalina's storybook get published as a children's book.
After Mario enters the library, Rosalina will begin to read the following story.
Chapter 1: The Celestial Duo
Chapter 2: Star Bits
Chapter 3: The Comet
The storybook appears in the secret ending of Super Mario Galaxy 2, where Rosalina is seen closing the book, having just finished reading it to the Lumas.
Super Mario Galaxy Trading Card description
In the Library, Rosalina reads a story to the Lumas about how a young girl and a Luma built a spaceship to go in search of the Luma's mama. Eventually the little girl takes on a motherly role for this Luma and count-less others, much like Rosalina herself.