What is good business writing?

An effective business text is one that enables the reader to be effective. If you enable your reader to be effective, you are by definition an effective writer;
An effective business message, text or document is one that causes the right thing to happen, in the right way, at the right time.

With the right tools, business people can achieve this in English immediately, whatever their native tongue is. We know this, because we've watched it happen for over 20 years.