I don't know what it is about these books that make they interesting. You wouldn't think that a series about four ex-friends who are being stalked by someone by the name A would be all that exciting, but they keep your attention.
Basically the plot for this one is the same as the other ones, but the ending was good. One of the girls gets hit by a car and might die, while another is being sent away for something her parents want to believe isn't real (which really pissed me off. How dare parents send their child away for being what they consider un-normal), one is homeless, and the other realizes that she might have killed their friend Alison. Nice cliffhanger.
So yeah, if you liked the other books in the series, you'll probably like this one as well.