Pride - Robin Wasserman

Adam and Beth are so over. And before their relationship is cold in the ground, they've each hopped into bed with someone else - namely, the "friends" who plotted to break them up in the first place. Harper and Kane finally have everything they ever wanted, and as they figure it, that's exactly what they deserve. All's fair in love and war - and to the victor, the spoils!
But it's not a happily-ever-after kind of world, and secrets can't stay buried forever. All it takes is one misstep, and the cocky Kane is just the right guy to make it. So much for happy endings.