The idiom often carries emotional weight, indicating strong approval or disapproval from someone. After arguing with his coach, he was in her bad books.
Pro Tip 2/3
Social Relationships
Used mainly in casual or social contexts, rather than formal or professional situations. She shared her lunch with me when I forgot mine, so I'm in her good books.
Pro Tip 3/3
Not Literal
There are no actual 'books'; it's a metaphor for someone's perception of you. Doing chores without being asked put him in his parents' good books.