How good is this extracurricular

Is writing a book and getting a publishing deal with a big four publisher good enough for the Ivy League?

Congratulations; getting a book published is a great accomplishment! Admissions decisions are very difficult to predict, and most schools will take a holistic view of your application into account. This includes the level of rigor of the courses you took in high school, grades, extracurriculars, test scores (if the school is not test blind), essays, recommendations, etc. Ivy League schools tend to have extremely low acceptance rates, making their admissions decisions even more difficult to predict. It is virtually impossible to say with any certainty whether a given student will be admitted to these highly selective schools.