Serena was mad because Dan's book portrayed her as a shallow, self-obsessed party girl. And flighty and irresponsible. Well, let's see here. In just this season alone she didn't think twice when asked to buy her boss drugs, she made some pretty judgemental comments about the small size of Charlie's apartment and tonight she insisted on complaining to Dan instead of letting him go to Blair, whose engagement was almost ruined. Yeah, I'd say her character was pretty accurate.