Shepard, Sara. Pretty Little Liars. 2006.
I just discovered this new series and am totally hooked. This series follows 4 girls that live on the Main Line in Philadelphia, which is a fancy suburban area. Years ago, they all were friends along with a fifth girl who mysteriously went missing. Alison had dirt on each of the other four girls, so when she disappeared, it was almost a bit of a relief to them even though they missed her. After her disappearance, Aria moved away and the others drifted apart.
Now, they are well into high school and Aria is moving back into town. Each girl starts getting text messages from someone who seems to know everything they are up to, and knows all of their secrets. Is Alison alive and stalking all of them or does someone else know the truth about everything?
Anyone reading the first book has to continue on with the series because there is a cliffhanger. There is enough juicy stuff going on to keep readers interested to see what will happen next. The only problem is probably the same with all other series-- the wait in between books!