Keeping up with the booming database industry isn't easy. There are hundreds of vendors, ranging from "on-line supermarkets," which offer 100 or more databases to "boutique" vendors, which carry only a few, specialized ones. The Fall 1983 issue of Directory of Online Databases, published by Cuadra Associates Inc., lists about 270 on-line services carrying nearly 1,800 databases produced by 900 organizations. Cuadra's directory, published twice a year with two supplements, costs $75 a year (2001 Wilshire Blvd., Suite 305, Santa Monica, CA 90403,  829-9972). A useful, but more selective (and somewhat out-of-date), listing is the 1982 Databases for Business, by Van Mayros and D. Michael Werner (Chilton Book Co., Radnor, PA 19089; hardcover, $27.50; paperback, $19.95). Two news-letters also help users keep up with new products: Database Update, for $97 a year (10076 Boca Entrada Blvd., Boca Raton, FL 33433,  345-8112 or, in Pennsylvania,  662-2444); and Database Alert, $48 a year (Knowledge Industry Publications Inc., 701 Westchester Ave., White Plains, NY 10604,  431-1880). Unfortunately, there is, as yet, no on-line directory of on-line databases.