I have read this book twice. This book is about a girl who babysits a neighbour's baby, and saves the baby from a serial killer. She confronts the killer, but due to the shock/trauma of being in that incident she forgets the face of the killer. She seeks psychiatric treatment to find the killer.
But the killer is really
This book has a purple cover featuring a house. The title may be The Dark or similar. I have searched countless books regarding this name but found nothing on it.
Some more details about the book:
The attempt of the murder took place on night with clouds and lightning. Not sure if there was rain. The killer had a pistol in his hand and was descending down the stairs, when the girl sees his face in a flash of lightning. Earlier, the girl goes into the upper room when the lights are cut off by the killer. The baby she later saves is in the living room downstairs. She remembers that the baby is in the house and rushes downstairs. I think she takes the baby and is going to go hide upstairs when she sees the killer.
This book was probably from 2002-2010, it was an American teen fiction novel, set in the 21st century America. I read it in American English.