Party Princess by Meg Cabot is the seventh book charting the (mis)adventures of Princess Mia, in The Princess Diaries. Party Princess was previously published as Seventh Heaven.
Poor Mia Thermopolis. Not only has she made a total ass of herself with J.P. (aka the Guy Who Hates It When They Put Corn In The Chilli) by trying to prove that she's a super-chilled party girl, but she's also bankrupted the student council. Way to go, Princess.
Just as Mia's scared that she's lost Michael and a ton of money, Grandmère steps in with a fundraising plan. She's going to stage a musical in front of the world's hottest celebs - and the star will be none other than Princess Amelia Mignonette Grimaldi Thermopolis Renaldo!