All I Want is Everything (Gossip Girl #3) by Cecily von Ziegesar
It’s Christmastime and Blair and Serena are best friends again, and up to their old tricks — partying hard and breaking hearts from Park Avenue to the Caribbean. Blair’s mom and Cyrus are having their honeymoon in Salt Key. And when school lets out for the holiday, Blair, Serena, Aaron, and company head down there to blow off steam after their midterm exams.
In between piña coladas and topless sunbathing, Blair and Serena plot revenge on super-jerk Chuck Bass. Everyone jets back to NYC for Serena’s New Year’s party, during which Nate and Blair may or may not finally go all the way . . . and Serena may or may not be discovered to be the secret fling of Hollywood’s hottest young leading man.