Remember Me Series by Christopher Pike

Shari Cooper hadn't planned on dying, but four floors is a long way to fall. Her friends say she fell but Shari knew she had been murdered. Making a vow to herself to find her killer, Shari spies on her friends, and even enters their dreams. She also comes face-to-face with a nightmare from beyond the grave. The Shadow - a thing more horrible than death itself - is the key to Shari's death, and the only thing that can stop her murderer from murdering again.

She won’t let them forget… Shari Cooper wakes up dead. The last thing she can remember is falling from a balcony during her friend’s party. Her death has been ruled a suicide, but Shari knows she was murdered. All of her closest friends are now suspects. As she tries to find her killer from the other side, she discovers her friends may not have been so loyal to her after all. Now, Shari is not just out for justice, she’s out for revenge….