One of four new ``Portraits of the Nations,'' by the author of excellent books on China (1989) and Mongolia (1990) in the series, this is a typically thorough, comprehensive survey of the geography, history, and culture of a land with several competing ethnic groups. Major clearly portrays the aspects and origins of cultural diversity and the resulting challenges the two governments here face. (Brunei, a small sultanate, ``declined to join'' when the country of Malaysia was formed in 1963.) The author's tone is rigorously objective except in a chapter on ``Daily Life,'' where fictionalized portraits of members of various cultural groups dramatize their differences. Elsewhere, the style is dry, even encyclopedic, more appropriate for consultation than for lengthy perusal. Still, a fine resource, the most complete on its subject for young readers. Many rather dark but well-captioned b&w photos; boxed ``mini-facts''; maps; tables; annotated chapter-by-chapter bibliography; index. (Nonfiction. 12+)