HOLY WEEK falls at the worst time in Central America's two-season climatic year -- at the scorched tail end of the dry, just before the rolling torrents of the wet. In the western, populated half of Nicaragua, the traditional torpor of the religious vacation is made worse by a constant swirling of the dust from miles of unpaved streets and long- denuded hillsides.

In Le,on, Nicaragua's old colonial capital and seat of Spanish Catholicism, everyone with money and transport leaves town for the beach or the mountains during Holy Week. For the rest, there is little to do. The prostitutes are off duty for religious reasons, the bars and businesses are closed. Blanketing everything is the heat. The sidewalks are so hot that the mongrel dogs of Le,on trot endlessly from one end of town to the other, their eyes glassy and tongues lolling.

It was during Holy Week in 1968, Omar Cabezas writes, that he was recruited into the Sandinista National Liberation Front. The invitation came from Juan Jos,e, a school friend remarkable for the amount of brilliantine he wore on his hair, and his unfashionable pleated pants.

"I knew it in my gut, sooner or later this had to happen; I'd heard talk of it I don't know how many times. Especially from the Social Christians, the professors, the fathers who sat down for a heart-to-heart talk with their daughters and sons who were coming to study in Le,on and live in big fashionable houses and eat lunch at Mama Concha's. They would warn their kids to steer clear of politics. Because politics gets you nowhere but into jail or the cemetery. Because politics is for grownups, not half-baked kids with no job and no income. Above all, they should not get mixed up with the people in (student politics), who were sympathizers of Russia and Fidel Castro. Besides which all communists were atheists . . . whose sole purpose was to send people into the mountains to be slaughtered like sheep."

In 1979, 11 years after Cabezas' recruitment, the Sandinista Front had overthrown Nicaraguan dictator Anastasio Somoza and taken power. Cabezas, now 34, serves their government as political director of the Interior Ministry.

Perhaps if Somoza, or the U.S. Central Intelligence Agency, had read a book like Fire from the Mountain, the Nicaraguan revolution never would have taken place. Told through Cabezas' early experiences with the Front, it is a primer on how Third World revolutionary movements are organized, how they recruit and incorporate activists among the general population, and how they develop a single-minded, almost superhuman dedication within the militants at their core.

There is little here that will add to the current debate in Washington over Sandinista intentions in Central America or the threat their government may or may not pose to the United States. No secrets are revealed as to how the Sandinistas got their weapons, or what their relationship was or is with Cuba and the Soviet Union.

First published in Nicaragua in 1982, it is hailed in its U.S. publicity as a major political event. But don't let that put you off. There is mercifully little -- outside a somewhat mushy introduction by Carlos Fuentes -- beyond passing reference to the usual staple of Nicaraguan revolutionary memoirs, the perfidy the United States and the evilness of its puppet Somoza.

IN AN INTERVIEW last year, Cabezas said he talked the book into a tape recorder, rather than wrote it. It is sometimes repetitive, frequently off- color and occasionally self-conscious and starry- eyed. Apparently little edited, it reads like a stream of consciousness, beginning during the week of his recruitment and following his experiences for the next eight years, ending long before the civil war actually began. Katherine Weaver's translation maintains the choppy, exhuberant quality of Nicaraguan Spanish. His early days in the movement, Cabezas says, were spent on the thousand small tasks of the revolutionary gofer -- finding safehouses for higher-ups passing through town, getting cars repaired, spying on the girlfriends of government soldiers. By the early 1970s, he and his fellow recruits had taken over the student movement of the Autonomous University of Le,on, and that is where their first real urban organizing took place.

"The harder we worked, the more students came out and were organized into study circles, which were then organized into teams, teams that later became cells of the (Sandinista Front)."

The student groups then moved into the poorer barrios of Le,on, in Cabezas' case into the Indian enclave called Subtiava. There, they learned about the Indians' own heroes and in a very calculated way began to tie those heroes with Augusto Cesar Sandino, the 1920s anti-U.S. fighter who gave the Sandinistas their name. The student workers found out what the Indians needed -- electricity, water -- and helped them form committees and elect leaders to make their wants known. Years later, those committees formed the basis of the underground networks and barricade-builders who helped win the war.

For those working in the cities, the guerrillas in the mountains were the real heroes. They had visions of vast legions of such fighters, just waiting for the moment when they would march down and take power.

Cabezas' account of his eventual trip to the mountains is both humorous and painful in its description of a soft, city-bred boy who found himself in a world of mud, rain and cold. The guerrillas had no food except for the powdered milk they could carry and the monkeys they killed. They fell prey to unspeakable diseases, carried heavy packs full of bullets and supplies through nearly impenetrable jungle, yet rarely if ever met the enemy. Worst of all, he found that the number of real guerrilla fighters resident in the eastern Nicaragua hills could be counted on his own toes and fingers, with some digits left over.

"I went with the idea that the mountain was a tremendous power. . . . It was where our power was, and our arms and our best men; it was our indestructibility, our guarantee of a future. . . . But sure enough, the reality hit, and you were right on the verge of demoralization when you got into the mountains and found that there was nobody there but Modesto with fifteen other men divided into little groups. . . . It made you want to turn right around and go back . . . You are right at the point of saying to yourself . . . this is the worst decision I've ever made in my life."

He returned after a year without ever firing a shot in anger, never having even seen Somoza's National Guard soldiers stalking them through the mountain jungles. He lost his girlfriend, missed seeing the birth of his child, and suffered a tormenting bout with mountain leprosy. But the small core of Sandinista guerrillas had achieved their purpose in making Cabezas and the few others who came with his unit into men like themselves, who were willing to be patient until their numbers could grow, who would not give up, and who were forming very definite ideas about what kind of a country they wanted to build.