Secret Smile by Nicci French
Miranda meets Brendan and have a quick three week relationship which ends after Miranda catches him reading her diary. Miranda isn't bothered by the break-up because she didn't really like Brendan all that much, but Brendan on the other hand was bothered. He was so bothered he starts dating Miranda's sister and well things go down hill from there.
I have to say I was disappointed reading this and can't really recommend this book. Well, I gave it a rating of "OKAY" because it was a quick read and I don't feel like I wasted too much time with it, but the ending is so half-assed it makes me wonder if the author even thought the plot out.
Oh yeah, they made a TV movie out of the book and well, that was pretty bad too.