L8r, g8r by Lauren Myracle is the last book in the "ttyl" series.
Through their instant messages, the "winsome threesome"-Angela, Zoe, and Maddie-have battled the ups and downs of high school. Now they're seniors, ready to rule the school. Unfortunately, they also have to deal with queen bee Jana, who has it out for good girl Zoe. Not that Zoe, who's deep in love with Doug, seems to notice. So it's up to Angela and Maddie to defend their friend, engaging in a series of pranks that escalates at a senior prom that no one will forget, even if they want to!
Lots of sexual content that they don't warn you about, so be warned! Other than that I found it to be a good read and a nice ending to the TTYL series.