Predicting New Words: The Secrets of Their Success, by Allan A. Metcalf. Houghton Mifflin, 208 pages, $22.

Metcalf, an English professor at MacMurray College in Illinois and author of several books on writing and language, is a delightfully amusing enthusiast for the richness of words and their use. Here he traces the never-ceasing expansion of English through the acceptance of brand-new -- or hitherto obscure -- words and phrases. Muggle, Y2K, dot-com, senior moment, drive-by are recent examples. Metcalf has invented, and here extends, a formula test for the likelihood of a neologism becoming widely used. He calls it the Fudge test, giving points for Frequency, Unobtrusiveness, Diversity of users, Generation of forms and meanings, and Endurance. Score your own. A volume that's lots of fun to dip in, and could be contrived into a rainy-day game or two.