The response code for a delete call can be any of the following :
DELETE /api/book/122– The server successfully processed the request, but is not returning any content204 No Content
DELETE /api/book/122– Resource does not exist404 Not Found
DELETE /api/book/122– Resource already deleted410 Gone
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DELETE /api/book/122– Users does not have permission403 Forbidden
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DELETE /api/book/122– Method Not Allowed405 Method Not Allowed
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DELETE /api/book/122– Conflict (User can resolve the conflict and delete)409 Conflict
In your case 404 is apt.