I do find it difficult to believe that your daughter spends "all day" on the gore-soaked computer sites or reading these books. I would assume that she would have to devote a fair amount of time to her advanced placement courses if she were succeeding academically. I'm sure that you mean that she spends an inordinate amount of time on the computer, looking exclusively at these similar sites.
I'm pleased that she has three friends. I am concerned that you believe them all to be very negative about everything. Do the other girls surf the same computer sites and read the same books as your daughter? If so, their friendship could be based on these similar attractions.
We also must consider the following possibility -- we may have four academically bright girls who have found a way to separate themselves from the rest of their classmates and gain a certain notoriety by immersing themselves in what other kids would consider bizarre "content areas." A cynical, pessimistic attitude about life is certainly shared by most kids at some time during their adolescent and early adult development.
Have you ever had open-ended discussions with her about the appeal of these websites and these books? If not, I'd have some, always being careful not to act like an interrogator looking for clues to a crime. I'd also consider setting some ground rules and limits on computer time. The computer, as she's using it, should be treated no differently than the TV. Would you allow her to watch hours of mindless TV per day?
Stay emotionally connected to your daughter. Find out about what's going on in her world and in her mind. Share your world and your thoughts with her. When is the last time you and she went out and did something together -- a Sunday breakfast, a movie she'd like to see, a long walk?
I don't have any snappy answers for you but I hope my observations might result in your asking better questions. Thanks for your letter.