It was released in 2000 by Harper Collins Publishers, and later became a film of the same name starring Anne Hathaway.
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The Princess Diaries is the diary of Mia Thermopolis, a fifteen-year-old freshman at the fictional Albert Einstein High School (AEHS), a private school in New York City. Mia is a tall, flat chested, big footed socially awkward teenager who was raised by her liberal artist mother Helen in Greenwich Village, Manhattan. Her best friend is Lilly Moscovitz, a genius, and she is obsessed with her crushes, Josh Richter and Lilly's brother, Michael. Her parents had a brief fling and separated before she was born; she sees her father Phillipe Renaldo, whom she is told is a Genovian politician, mostly when she spends every summer at her grandmère (grandmother) Clarisse's French chateau, Miragnac.
Mia's world is thrown upside down when her father, who recently underwent treatment for testicular cancer, finds out he cannot have any more children. This fact forces him to tell Mia that he is not just some Genovian politician, but the Prince of Genovia. Because Mia is his only child, and his cancer has prevented him from having any more children, he informs her that she is now Her Royal Highness Amelia Mignonette Grimaldi Thermopolis Renaldo, the princess, and heir to the throne, of Genovia.
Mia does not adjust to this news easily; but she certainly objects when her father informs her she must now move to Genovia to learn to be a princess. Mia and her father work out a compromise in which she will take the throne as long as she is allowed to finish high school in New York. Eventually, Grandmère decides to come to America to provide "princess lessons," which her father offers to pay her for. Mia declines until her father says he will donate one-hundred dollars a day to Greenpeace in her name. Mia wants to keep her royal status secret, because firstly, she knows her best-friend Lilly is a virulent anti-royalist; and secondly, she does not want to be treated like a freak at school.
Throughout the book, Mia is infatuated with both the popular jock, Josh Richter, and her best-friend's brother, Michael Moscovitz. Mia's best friend, Lilly, finds that the deli, Ho's, near AEHS favors Asian-American students by giving them a 5-cent discount, and stages a ban on the deli. She films the petitions against Ho's on her public-access TV show, "Lilly Tells It Like It Is", which Mia would usually help with. Unfortunately, due to her new princess lessons, Mia is unable to help, and Lilly gets mad at her. Mia is forced to get a makeover by Grandmère, and when she goes to help edit the "Ho Gate" piece, Lilly sees her new haircut and fake nails, and tells her she is turning into popular cheerleader Lana Weinberger, Mia's worst enemy. This starts a fight between the two friends, and Mia refuses to say sorry because she wants to stand up for herself.
The next day at school, Mia cannot sit with Lilly at lunch because of their fight, so she asks to sit with Tina Hakim Baba, who has no friends. Mia and Tina quickly become friends, and Mia feels bad for never having been nice to her before. The whole of AEHS considers Tina a freak, because she has a bodyguard who follows her around by orders of her father, an Arab sheikh. When Lana makes fun of Tina, Mia lashes out against her and throws an ice cream cone at her, which results in a visit to the principal's office.
The next day at school, Josh Richter says hello to her, and students giggle whenever they see her. Mia is confused until she finds out that an article in the paper announced her position of royalty. Her father visits the school, and tells her that she must be followed around by his bodyguard, Lars, so she will be safe. In classes, Lilly attacks the Genovian government, but Michael, who is tutoring Mia in algebra, defends her. At lunch, Lana Weinberger, who is dating Josh Richter, attempts to befriend the princess, but Mia and Tina stick together. Josh quickly breaks up with Lana, and asks Mia to go with him to the upcoming Cultural Diversity Dance. Mia accepts immediately, even though her better thought tells her not to.
With the help of Grandmère, Mia convinces her parents to let her go to the dance. When they get there, Josh leads Mia in front of the press, and kisses her. Mia realizes then that he was only using her to get into the papers, and ditches him for her friends at the dance. Once she apologizes to Lilly, their friendship is restored, and Mia spends the night dancing with Michael. She sleeps over at the Moscovitzs' house, and due to a more important news story, the kiss never gets in the papers, saving Mia a scolding from her father. At their house, Michael plays Mia a song he wrote, called "Tall Drink of Water".