Merchants of Grain, by Dan Morgan. New York: Penguin Books. $3.95.

Continental, Cargill, Louis Dreyfus, Bunge, and Andre -- don't feel bad if you don't know anything about these giants of the grain industry; they don't want you to know anything about them. All five have done their best to limit public knowledge of their movements. Dan Morgan, clinging to the scrubbiest, scarcest sources, explores how these huge conglomerates discreetly influence domestic and foreign policy -- food-related and otherwise. In this interesting book on what seems like a boring topic, the reader tiptoes with Morgan in the revealing pursuit of the enormously potent grain companies.