The Guardians — Lynn Abbey

Horror: This book is S C A R Y!
There's a witch on Riverside Drive...
Lise Brown was thrilled with her new apartment; it was more than anyone fresh from the Midwest had a right to expect in Manhattan. She loved it for the river view and the high old ceilings, but even more for the unexpected way she felt instantly at home there - as if the place had been waiting for her.
She could not know that apartment 647 contained a rift into another world, a frightening world of malevolent spirits kept contained only by the rituals of the previous tenant. She can feel the evil on the other side of the rift, growing stronger, feeding on her fear, waiting for the moment when it will be powerful enough to break through into a helpless world. To stop it, Lise must learn what it means to be a witch...