Remember Me (Vintage Paperback) 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.