The American Management Association (AMA) was founded in 1913 as the National Association of Corporation Schools. In 1973, five closely related national associations were consolidated into the AMA, which is recognized as an educational institution. It is now a global institution. They publish a book that is available called the AMA Style guide for Business. It can be purchased on Amazon, Barnes & Noble and other book websites.
The University of Oregon has put out a guide for business writing that can be found at: http://pages.uoregon.edu/ddusseau/101/199/style.html Another good source is Alexander Communications’ style guide, found at: http://www.alexcommunications.com/style_guide.htm