I recently ran across a useful little book, The Elements of Visual Style: The Basics of Print Design for Every PC and Mac User, by Robert W. Harris. While it's aimed at print-based writing, webwriters can also draw some lessons from it.
Harris gives us a quick guide to typography, layout, and the use of art in print documents. The illustrations show bad and good examples, and the book itself is pretty well designed. I wish it were more "hypertextual": We get no references to other books on document design, and no links to sites dealing with this and related issues.
Still, it's a compact, concise, and inexpensive handbook. Even if you find most of the advice very familiar, the book could help you back up the points you're trying to make to your clients.