Aimed at middle-graders, this look at the place of consumers in our economic system is so full of theory and general statements that it makes for heavy reading. The author describes how consumers regulate a market economy, as well as how they influence both political currents and social structures and expectations. She introduces concepts such as ""merit"" and ""demerit"" goods, ""opportunity costs,"" the ""maximizing consumer,"" and ""checkable"" (i.e., checking) accounts; mentions ATM's and electronic funds transfers; and pleads with consumers to read labels, research large purchases, and make intelligent choices. She disapproves of using credit, which is defined here as ""spending tomorrow's income today."" Specific examples, however, are few and unimaginative--""you"" have to decide whether to buy ten cassettes or those designer jeans; ""Jane"" illustrates supply-and-demand by buying more meat when the price goes down. A good discussion of how the GNP is computed suffers from a lack of precise results; indeed, many of Killen's points are dissipated in generalities. No pleasure to read, but this may fill a need where encyclopedia articles are not acceptable for assignments. Photos (some in color), index, glossary.