I love Meg Cabot (Heather Wells & Queen of Babble) and Sophie Kinsella's (Shopaholic [soon to be movie] & Remember Me) Emily Giffin (Something Borrowed & Something Blue, Baby Proof) books. I am in the process of reading Jennifer Weiner's (Good In Bed & Certain Girls) books. They are all great authors!
For a young adult/teen Meg Cabot's young adult books. She is the best author. Also best selling. She wrote tons of books including the Princess Diaries (Movies based on her books), Mediator (upcoming movie), 1-800-Missing (TV show based on her books), Jinx, Airhead and more. She has adult books too, so you can grow with her.
If you want more info or any of the authors website's email me.