The fifth in the Princess Diaries series by Meg Cabot. I really like this one -- it's more dramatic than books two three & four, and very funny.

I love it that Mia and all her friends are full-out geeks, but they're not traumatized by it, except when dealing with awful highschool hotties like Lara. The rest of the time they're just geeking out, having parties and organizing protests and practicing the violin. And even Boris the mouth-breathing, sweater-tucking-in mega-geek turns out to be a romantic hero -- to more than one girl!

I haven't read Teen Idol yet -- Cabot's newest. But I liked American Girl a lot, so I'll definitely steal it from my little sister as soon as she's done with it. My modus operandi is to buy teen books for her as presents, then snag them for myself once she's finished.