The DUFF (Designated Ugly Fat Friend) by Kody Keplinger

High school senior Bianca has two gorgeous, wonderful best friends: Casey, tall cheerleader captain with a short pixie cut of blond hair, and Jessica, super bubbly brunette. Bianca is a bit cynical herself, and doesn't fully enjoy going to the club that her friends always drag her to, but does it because she loves them (and to keep them from going home with someone like Wesley Rush--the school's biggest womanizer).
One night at the Nest, the club Casey and Jessica drag Bianca to, Wesley Rush approaches Bianca. She tells him to get lost but he says he's trying to get with one of her friends--it doesn't matter which--and that by talking to their DUFF, he has a much better chance of doing so. Bianca has never heard that term before so Wesley obligingly explains: the DUFF is the Designated Ugly Fat Friend, the one girl in a group of girls who looks worse than the others. And by paying attention to the DUFF, Wesley reasons, he'll look like a great guy to Casey and Jessica. Bianca, never one to hold back, throws her Cherry Coke in Wesley's face and drags her friends out of the club.
She doesn't tell her friends what has happens between them and it's only one of many things she keeps from Casey and Jessica: her parents are divorcing (her mother has been largely absent anyway), her father slips back into his alcoholism (after being sober eighteen years), and Bianca is trying to find out how to relieve the stress, how to keep from going crazy. She discovers her escape is actually Wesley Rush--making out with Wesley keeps her mind from thinking about her parents. She spends more and more time with Wesley (and both keep is a secret) and Bianca's friends begin to feel she's avoiding them. She never likes to talk about herself or her problems much (Bianca's standard answer is "I'm fine" and Casey knows she's lying every time she says it). Bianca gets deeper and deeper into her struggle and her world continues to unravel when her mom shows up. She keeps running to Wesley and after one hook-up, she realizes that it's more than just screwing around--for her. Wesley's not interested in a DUFF so Bianca tries to forget him and hooks up with her crush, sweet, dorky Toby Tucker. Although things are approaching normal, Bianca still thinks about Wesley and being the DUFF.
I think all teenage girls have felt like the DUFF amongst their friends, and Keplinger does a great job of having the characters take control of the word and have confidence in themselves and each other. DUFF is a good read (definitely for older teens), and I love Bianca's character. She's balanced well by Casey and Jessica and it's nice to see confident but still vulnerable teens. The DUFF comes out in early September.