One more note about the extended universe of Lois Duncan before I drop the subject.
Remembering "I Know What You Did Last Summer" prompted me to look it up on IMDb to find out who played Sarah Michelle Gellar's sister. It turns out it's Bridgette Wilson. It also turns out that "I Know What You Did Last Summer" is notable because though it centers around four main characters, Freddie Prinze Jr. and Sarah Michelle Gellar's characters never speak to each other. They're in a ton of scenes together, but never a word directed between the two. It's a cinematic oddity.