After a little search, I still haven't found a teenage-specific book about BPD (no wonder, since they don't want to officially diagnose before 18 either... .).
However, I have read this little book, and thought it was an easy read, full of good info, but not overwhelming. It might work well for a teenager:
Borderline Personality Disorder: A Guide for the Newly Diagnosed, by Alexander L. Chapman PhD RPsych (Author), Kim L. Gratz PhD (Author)
www.amazon.com/Borderline-Personality-Disorder-Diagnosed-Harbinger/dp/1608827062It's not expensive, but if you are not sure, you might want to check it out at your local Library, and if they don't have it, then through their inter-library loan program and see if you like it enough to buy.