In the declarative sentence:
George became a writer of detective stories.
We changed it to interrogative sentence:
(What / Which) did George become a writer of?
Should I use What or Which?
You only use "which" when you present a choice of answers. For example, if you presented someone with an apple, a banana, and an orange, you might ask "which would you like?"
As yours is an open question, you should use "what".
A more formal construction of the question would be:
Of what did George become a writer?