If you have ever seen the "I Know What You Did Last Summer" movie, don't presume you know anything about the book. It is vastly vastly different... one is a teen thrilled and the other a slasher movie. In the end it's even a different person who "does" it. Different people end up dead. It's like a completely different story. And as such it is well worth reading.
Lois Duncan has a keen insight into the teenage girl and it's always entertaining to read her heroines and see the way the interact with others and the way they deal with the situations thrown at them. Not my favourite of Lois Duncan books, but good all the same.