Don't Make Me Think by Steve Krug
A well-designed, easy-to-read, and extremely helpful book about Web design and usability.

How Winners Sell by Dave Stein
A great book on how to sell, especially big-ticket items and complex services.

Influence: The Psychology of Persuasion by Robert Cialdini
I love this book. It explains why some marketing strategies are so effective.

On Writing Well by William Zinsser
A well-written book on writing (which is more unusual than one might think). I love all of it, but particularly enjoy Chapter 11 ("Writing About Places: The Travel Article").

Promoting American Engagement: A Catalog of Recommended Frames and Language by Axel Aubrun and Joseph Grady
An article describing the Global Interdependence Initiative of the Aspen Institute. Very helpful for NGOs. Available at http://www.frameworksinstitute.org/products/global.shtml

Selling the Invisible by Harry Beckwith
A book full of concise gems about selling services.

The 22 Immutable Laws of Marketing by Al Ries and Jack Trout
A skinny book with fun examples of marketing campaigns.

The Chicago Manual of Style, 15th Edition
The style manual I prefer.

The Cognitive Style of PowerPoint by Edward Tufte
An intelligent indictment of the most popular business presentation tool.

The Mind and The World: Changing the Very Idea of American Foreign Policy by George Lakoff
A thought-provoking article available at http://www.frameworksinstitute.org/products/global.shtml

The Tipping Point by Malcolm Gladwell
An interesting and popular book.

The Visual Display of Quantitative Information, Envisioning Information, and Visual Explanations—three books by Edward Tufte
Tufte is the guru of this subject, and these are beautiful, dense, illuminating books for those doing visual and data communication.

Words Into Type, published by Prentice Hall
Covers what the Chicago Manual of Style doesn't. What I use to make my case with designers about the proper typesetting of ellipses, en dashes, and em dashes. For the pickiest of the picky.