For general marketing information, check out the American Marketing Association web site and/or give them a call (312-648-0563). Also check out Mouse Tracks, which "presents and offers commentary on some of the marketing activities and resources on the Internet."
The leading treatise on marketing is Principles of Marketing by Philip Kotler. Another option: Business: The Ultimate Resource.
There are dozens of marketing journals on Lexis including Marketing Health Services (MARKET;JNLHCM), Journal of Consumer Marketing (MARKET;MARS) and Direct Marketing (MARKET;MARS). A different selection is available on Westlaw.
For definitions, the AMA posts a free Dictionary.