HARD COURTS By John Feinstein Villard Books 457 pp., $22.50JOHN McENROE, perhaps the most famous and feisty tennis player of the '80s, has set the stage for the '90s too. So argues John Feinstein in his book about the 1990 tennis season, "Hard Courts." When McEnroe was ejected (for swearing at an umpire) from the Australian Open as the year dawned, the chaotic scene surrounding his default "was a perfect metaphor for what professional tennis had become as the 1990s began. Tennis was in a period of turmoil and transition - again. The men were split politically, while the women had sponsor problems and were counting on a 13-year-old [Jennifer Capriati] to replace the irrepl aceable Chris Evert as both icon and girl next door." Despite blatant commercialism and infighting between players, tournament directors, and player-management groups, the sport carries on, Feinstein finds. Though tempted to "tank" a meaningless match, players usually don't. Good, even great, tennis is still played. The game, a reader sees, is filled with real people, people who sometimes do things good and worthy and other times sink into, well, stupidity. It's the lure of money that provides constant pressure to distort values. Of the 2,042 players who earned at least one point in 1990 on either the men's or women's computer rankings, "perhaps 200 were making a good living," Feinstein estimates. "Of those 200, about 50 - maybe - are extremely comfortable. About 15 are filthy rich." Yet the 2,042, he points out, are only the tip of an iceberg, "the elite among hundreds of thousands who dream about striking it rich in the sport." Ironically, it is two nonplayers who win the highest kudos from Feinstein. Their love for the game seems to refresh him when he feels jaded. One is the late Ted Tinling, who won fame as a designer of women's tennis fashions and became unofficial spokesman for the women's game. His keen British wit could bring down the haughtiest of players; his long memory put events in context. The other is Feinstein's tennis mentor, veteran writer and television analyst Bud Collins. Feinstein writes that Collins was once asked "How can you care so much about a sport with so many bad people in it?Collins just smiled," Feinstein narrates. "[Collins's] friends often say he would have found some good even in Hitler. 'Not Hitler, no, Collins answers. "Mussolini, maybe." Feinstein's four pages of acknowledgements in the preface show why he's a top reporter: sources, sources, sources. If anything's lacking, it's an index to help readers find passages about favorite players. For those who want a deeper understanding of pro tennis than their daily sports section provides, Feinstein has served up an ace.