The American Heritage College Dictionary of the English Language, 3rd ed., 1996. Good, standard desk dictionary; also available on CD-ROM.
Angell, David, and Brett Heslop. The Elements of E-Mail Style: Communicate Effectively Via Electronic Mail, 1994. Concise guide to e-mail rules, including style, tone, typography, formatting, politics, and etiquette.
Barron's Business Guides. Dictionaries of terms related to accounting, business, banking, investment, marketing, and so forth. Comprehensive dictionaries for various fields of business.
Browne, M. Neil, and Stuart M. Keeley. Asking the Right Questions: A Guide to Critical Thinking, 5th ed., 1997. Useful textbook on critical thinking; focuses on question-asking skills and techniques for evaluating evidence.
Brusaw, Charles T., Gerald J. Aired, and Walter E. Oliu. The Business Writer's Handbook, 5th ed., 1997. Extensive handbook for business writers; alphabetical entries cover grammar, usage, and guidelines for types of business writing.
Caruth, Donald L., ed, and Steven Austin Stovall. Taking Care of Business: The Dictionary of Contemporary Business Terms, 1996. Dictionary of over 4,000 basic terms, abbreviations, and colloquialisms used in banking, business, investment, and finance.
Fowler, H. Ramsey, and Jane E. Aaron. The Little, Brown Handbook, 7th ed., 1998. Indispensable reference/textbook for all kinds of writers at all levels of proficiency.