The young-adult book is set to go on sale Tuesday, along with a historic romance novel called "Ransom My Heart," supposedly written by the fictional "Diaries" heroine, Princess of Genovia Mia Thermopolis. In reality, Cabot penned the book under the name Patricia Cabot in the 1990s, but couldn't get it published at the time.

"It's bittersweet," Cabot told USA Today about wrapping up her "Diaries" series. "It's a lot harder than I thought it would be to say goodbye."

The books, which inspired two movies starring Anne Hathaway as the titular teen royal, follow American girl Mia Thermopolis, who, at 14, learns she is the princess of the kingdom of Genovia. The last installment in the franchise sees her write a romance novel and graduate from high school.

"I really didn't plan anything beyond when she was 18," Cabot told USA Today. "Every girl who writes me says, 'Mia's exactly like me except she's a princess. ... She really does have that Everygirl voice and Everygirl problems. Readers relate to her."

Cabot is also the author of the "Allie Finkle's Rules for Girls" and "Airhead" series of books.