Conversation so far:
who wrote the script for the film Rosmary's baby?
The film Rosemary's Baby was written by Roman Polanski, based on the novel Rosemary's Baby by Ira Levin.
What was Roman's Polanski's inspiration for the story?
The film Rosemary's Baby was written by Roman Polanski, based on the novel Rosemary's Baby by Ira Levin.

Question: Who wrote the novel?
Self-contained rewrite: Who wrote the novel Rosemary's baby?