June 29, 1986

ALONG its western boundary on the Hudson River, Yonkers is a fairly poor and rundown industrial community. But as it climbs the ridges of Westchester County, New York State's fourth largest city becomes increasingly rich, residential and polished. That diversity has led many people to describe the history of Yonkers as a tale of two cities, the well-to-do residential east and the down-on-its-luck industrial west. But in the last several years, the split between the two halves has narrowed as developers ran out of space in nearby communities they regarded as more atractive and began turning to Yonkers. Now this 18-square-mile community north of the Bronx and south of Hastings-on-Hudson is undergoing what might fairly be called a building boom. Under construction or in the planning stage are nine major projects that, if fully realized, will give Yonkers 2,500 new residential units and more than 4 million square feet of commercial space. Among them are a 2,000-unit waterfront condominium complex next to the Yonkers train station, a 1.5 million-square-foot office center with a hotel in the northeast section of the city and the community's first Manhattan-style condominium tower.

The Mets, who have been struggling a bit lately, scored three runs in the ninth inning today and beat the Chicago Cubs, 5-2. And they did it in style by getting the key hits off Lee Smith, who had stopped them cold by striking out three heavy hitters in a similar jam one week ago. The victory was the 48th of the season for the Mets against only 21 losses, the best record in baseball as the major leagues approach the halfway mark. It was also the seventh victory against no losses this season for Roger McDowell, the second-year relief pitcher, who became the first player in the 25-year history of the club to win seven games and lose none at the start of a season.

NICARAGUAN guerrilla leaders have big plans for the $100 million the House approved last week. Once the money is also approved by the Senate, as is generally expected in Washington, the rebels say they intend to spend it on weapons and training. The guerrillas, whose bases lie near the Nicaraguan border about 140 miles from here, emphasize that the two items they would like to receive as soon as possible are surface-to-air missiles and ammunition.

KATE VAIDEN By Reynolds Price. 306 pp. New York: Atheneum. $16.95. ''I'D caused their deaths.'' Every writer who has produced a large body of work has themes that play as relentlessly as a ground bass beneath the changing melody of plot and character; every book could yield up a single sentence from the center of the author's obsession. Reynolds Price has shaped many of his novels around the lives of children whose mothers have died giving birth to them, or whose conception is in some other way clouded - children, that is, who feel or are treated as if they are guilty from their first breath. ''Kate Vaiden,'' which teems with orphans and murderous and suicidal generations - all the expected passions of a Price book - is nonetheless different from the North Carolina novelist's other work. It is a forgiving, immensely readable story, set mainly in the early 1940's, almost light in feeling (although its tale of early death and frustrated passions is hardly frivolous). But the voice of Mr. Price's heroine blows like fresh air across the page. ''Kate Vaiden'' even looks different from its eloquent but frequently ponderous predecessors, typographically dense from margin to margin.

ONE Mexican newspaper found the headline that said it all after Argentina's quarterfinal victory over England in the World Cup soccer championship here last weekend: ''Maradona 2, England 1.'' But when the brilliant young Argentine midfielder, Diego Armando Maradona, scored his team's only two goals against Belgium in last Wednesday's semifinals, the same daily was reduced to the obvious: ''Maradona - Yet Again!!'' Such has been the excitement stirred by his performance here this month that, even before Argentina tries to repeat its 1978 triumph in the final today against West Germany, this has been baptized Maradona's World Cup. Such established players as Zico of Brazil and Michel Platini of France and such new talents as Careca of Brazil and Gary Lineker of England were all on hand, but the crown of the world's best player today goes undisputedly to the poor boy from the Buenos Aires barrio who found fame and fortune in soccer.