The Princess Diaries, Princess in the Spotlight, and Princess in Love (by Meg Cabot)
A cursory glance through Amazon tells me there are 10 books, which, yay! I loved the first three. It's much slower paced than I expected, most likely because I'd seen the movie, and a lot of action was crammed into it. But I love the main character, Mia, and the supporting characters (particularly Lilly and Grandmere, who are very vivid, and gives me some ideas about bringing my supporting cast to life). Anyway, most of my brain was spent just LOVING THE HELL out of these books; the rest of my brain was swirling with ideas for my own YA thingie.
I know I'm late to the P-Diaries party, but really. So much fun. And I feel far less guilty about liking these than the Shopaholic books...