Africa today -- a journalist's readable, cautiously hopeful account; The Africans, by David Lamb. New York: Random House. 363 pp. $17.95.

As an introduction to black Africa and its complex, changing problems, David Lamb's ''The Africans'' is first-rate. Splendidly anecdotal and highly readable, it recognizes the Africans' strengths - the emotional warmth and humor, the high morality, and close family and tribal ties - while also tackling their weaknesses - the hysteria and fear that can turn goodness into violence and hate.

The book is based on the author's four-year assignment as the Nairobi-based correspondent for the Los Angeles Times. Mr. Lamb managed to visit nearly all of the 48 enormously diverse countries of sub-Saharan Africa, a feat that wins the reader's sympathy and interest from the start.

Lamb has a talent for that happy choice of phrase that brings an instant flash of understanding: He describes his job as ''catching midnight flights to little-known countries where very nice people were doing very terrible things to one another.'' Too many African nations are led by ''men who are little more than clerks with guns.'' To Americans of African descent who go back, ''Africa may be the homeland, but the United States is home.''

He also has the good reporter's knack of being in the right place at the right time to tell the right story. We are taken to the Liberian beach where Sergeant Doe's firing squad murdered the Cabinet members; to the Emperor Jones-style crowning of another ex-sergeant, Bokassa; and to the bedroom of Idi Amin, just after the mad dictator fled Uganda. In that room Lamb notes the warplane pictures taped to the walls, hand grenades under the bed, pills to treat venereal disease, reels of old ''Tom and Jerry'' cartoons, a file cabinet stuffed with photographs of torture victims.

Many of the stories are tragic; a few are funny.

During a visit to Mary Leakey's dig in Tanzania's remote Olduvai Gorge, the legendary but crusty searcher for man's origins loses no time in letting her guests know that she likes old bones better than young journalists.

But Mr. Lamb's portrait of Africa, while not without hope, is essentially grim - the story of a continent ravaged by colonialism and change, cut off from its history.

Food production is falling, population rising; cities are turning into slums; prisons are jammed; and fully 5 million refugees have been driven from their homelands by wars, tyrants, and famines.

Indeed, just since ''The Africans'' was published, over a million Ghanaians and hundreds of thousands of other Africans have been quick-marched out of Nigeria, with untold suffering and hardship.

Technological common sense suggests that the years between 1970 and 2000 should be the period of the third world's great advance in solving its acute problems of food shortage, people surplus, and resource profligacy. Asia promises to do so. Yet the emotional folly that can be such a deadly characteristic in so many of Africa's leaders has so far prevented this kind of advance on that continent.

Just why is a mystery. Mr. Lamb's book puts together some of the jigsaw pieces - lack of skills and education, corruption, tribal favoritism - but it leaves too many missing.

One suspects that the demands of his newspaper assignment too often forced him to look at problems in terms of the politics at the surface. He says as much: ''Sometimes, for months on end, I bounced from wars to coups d'etat.'' Yet he found his most rewarding work came during quiet lulls ''when there was time to explore the cities and villages.''

This may explain why he sheds insight when he identifies the deeper economic and cultural trends (the African woman's disproportionate farming role), but tends to sound glib when he proposes solutions (a ''Marshall Plan'' for a continent that will be tutored by a somehow reincarnated, racially integrated South Africa).

A good many of Mr. Lamb's observations go against the grain of much recent American writing about Africa, but nothing evidently stops him telling his own version of truth, unpalatable as it can sometimes be.

''The Africans'' is not a scholarly, profound work. It is the work of an objective and sensitive journalist who went just about everywhere and saw just about everything and who came to care enormously about the fate of Africa's people. This book is badly needed, not least for its predominately hopeful tone. As David Lamb says, ''Africa's dreams have been only mislaid, not lost.''