I like Meg Cabot's Princess Diaries books: they're fun and fluffy and don't pretend to be anything more than that, really. Cabot's characters have good moral centers (gaak, cannot believe just wrote that) -- but really, they do. These books are a decent antidote to the soulless Gossip Girls and Clique series: it's the same kind of name-dropping, trend-heavy contemporary setting, but focuses on the dorks and misfits of the high school social scene.
That's not to say the formula can't get a little tired, though, and this (the seventh in the series) is showing some signs of age. Now that Mia has her perfect boyfriend, the tension of feeling like a romantic loser has abated somewhat; now she frets about taking her relationship with Michael to the next level (S-E-X). I did enjoy the intriguing little J.P. subplot in Party Princess: Mia doesn't quite realize she's flirting with (and enjoying being flirted with in return) another great guy, but the reader (and Lilly) sure does. I'll be interested to see if the Michael/Mia pairing survives another book.
A good choice for summer or when you've got two or three hours to kill.
Party Princess by Meg Cabot (The Princess Diaries, Volume VII). Harper, 2006, 304 pages.