Remember Me 3 by Christopher Pike

Someone did not like Shari Cooper writing her stories. Shari Cooper had died once, and then returned to Earth as a Wanderer -- a soul who had been given permission to take the place of another soul in a mature body. Shari has regained her memory of her previous life. More than that she has realized her purpose in returning to mortal life -- to write stories for young people to help them understand the immortal life that is to follow. Her talent is inspired, her destiny great, and it is not long before Shari and her books are known all over the world.
Then one special night a story of incomparable beauty and mystery comes to her. An ancient tale that speaks of the origin of mankind and the purpose of human life. Quickly, almost in a fever, Shari begins to write it down, sure that it is nothing more than a wonderful fable. But what Shari doesn't know is that her new book is true -- a mystical blueprint that warns of a great danger to humanity from creatures who despise all human beings, but who have a unique and terrifying hatred for Wanderers. Creatures who will go to any length to stop Shari's story from being published.

Creatures who wait unseen outside Shari's door.