What is the HTTP response code for failed HTTP Delete operation?

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 content
  • 204 No Content
• DELETE /api/book/122 - Resource does not exist
  • 404 Not Found
• DELETE /api/book/122 - Resource already deleted
  • 410 Gone

• DELETE /api/book/122 - Users does not have permission

  • 403 Forbidden

• DELETE /api/book/122 - Method Not Allowed

  • 405 Method Not Allowed

• DELETE /api/book/122 - Conflict (User can resolve the conflict and delete)

  • 409 Conflict

In your case 404 is apt.

Yes, it would be 404.

In general it will be a 400 series error if the request is wrong somehow, and a 500 series error if something goes awry on the server.