writingcommons.org is a free and easy collection of articles about the issues that concern writers in the new media mileu.
If you skip to the Rhetorical Analysis section, you’ll find the system to follow for choosing words and graphic design that work to persuade the audiences for your business communications.
Be sure to read the section on visual literacy. The new Rhetoric incorporates visual communication formally into the body of knowledge that previously, from Aristotle forward, concentrated exclusively on speech and writing.
I’ve been arguing for (and teaching) visual analysis for nearly thirty years, so I know for a fact that the field of Rhetoric advanced conservatively in an environment of radical change.
Until human nature becomes resistant to persuasion, the principles (canons) are still the organizing structure to use. However, you are on your own when it comes to using it to discover what works for the immediate situation and particular audience you have to work with. The “Next New Thing” is history by the time Art and Science describe it.