Judy Blume's "It's Not the End of the World" is a novel that could be best categorized as falling within the genre of young adult fiction. Blume's work is generally aimed at young adults, or teenagers, going through some stage of transition in their lives.
The main character of the novel is Karen, who is seeing her parents' marriage collapse. She keeps a journal where she rates each day like a school paper: A+ to C-. The novel itself is somewhat autobiographical, though it isn't obviously so—it merely reflects some emotions the author was going through during her own divorce as she reflected on her children.
Karen holds on to the idea that her parents' marriage can be saved, and does everything in her power to try and make that so, unsuccessfully. The climax of the novel comes when Karen's brother Jeff, out with the family for dinner, has an argument over what he ordered, and runs away.
When Jeff doesn't return, Karen's parents, rather than being united and concerned for their son, fight bitterly with one another. At this point Karen sees clearly that the marriage is doomed, and is able to begin to accept that reality.