Not talking about your style, prose or grammar, but how you structure the points you want to come across. It's not just useful in the context of "business writing", it's useful whenever you want something from someone else: to take an action of any kind.
I see it every week at work where people put a presentation together at first glance seems to be coherent, but if it had been written out as a document / memo, you'd easily see the gaps in their thinking. Forcing yourself to write out full sentences (not bullet points) into paragraphs and those paragraphs having a logical connection to each other, shines a spotlight on places where your thinking is weak or there are unfounded conclusions.
Here's the good news: there's one book that will teach you this thoroughly: Barbara Minto's - "The Pyramid Principle: Logic in Writing and Thinking". Bad news: the book is out of print, since the author wants people to either buy her courses or her updated textbook which she charges $150 for (not a typo): http://www.barbaraminto.com/textbook.html
The book is insanely useful, so find a used copy of the 1987 or 2002 edition in a used bookstore.