Anyone know what the latest edition of The Gregg Reference Manual is?

Answer 1

Its full name is:
The Gregg Reference Manual: A Manual of Style, Grammar, Usage, and Formatting (Gregg Reference Manual)

That says it all, eh?

You can buy the spiral-bound 10th edition at Amazon:
http://www.amazon.com/exec/obidos/tg/detail/-/0072936533?v=glance

Publisher McGraw-Hill calls it "your indispensable guide to business writing."

Hope that helps.