Mediator 6: Twilight -- Meg Cabot
First off, if you haven't already started reading these books, DO! They're like Buffy, except that Suze deals with ghosts instead of vampires. She's pretty fashion-obsessed, so she gets really, really mad if the ghost (or living person) she happens to be fighting with messes up her Jimmy Choos.
So the Princess Diaries books have become the lamest of the lame. Big deal. Mag Cabot still has it. Twilight was AWESOME. Awesome awesome awesome. A.W.E.S.O.M.E. Kickass. Rock on.
Four words: Mediators can time travel.
I'm a little worried that this might be it, though. The next book in the series (if there is one) will be make-or-break. I'm actually not all that hopeful--this will probably, sadly, be the last good one. Oh well. I just ordered the first five books in the series, so I can just re-read these six to my heart's content.
(And yes, I know that I already own most of them, but a) they're all still packed up, and b) I wanted the ones with the new covers).