JOHN FEINSTEIN is on the move again.The author of the best-selling sports book of all time is on the phone from somewhere in New York City. The sounds of city traffic - beeping horns, squealing brakes, whining transmissions - punctuate the conversation. "Season on the Brink" made a loud noise of its own a few years ago. Feinstein spent a season following the University of Indiana basketball team and its coach, Bobby Knight. His look at the inner workings of a major college program won praise from many; others found it too candid. Now Feinstein has turned his attention to his other sports love, tennis. To write his new book ("Hard Courts," review below), he traveled for a year on the men's and women's professional circuit, piling up frequent flyer miles and notebooks full of interviews with players, officials, and coaches. Why a book on tennis? In part, Feinstein says, it was the challenge involved. "It's a damn tough sport to crack" as a journalist, he explains. "It's tough to get [to] the players. All the big ones now have their agents and their entourages. It's one of the few sports where you have no access to the locker room, so striking up casual relationships with the players is difficult." Though, in general, women players are easier to arrange interviews with, they often have less to say "because they're so young, because they're so inexperienced, because most of them are uneducated," Feinstein says. But reporters who lament that today's teenage phenoms are less articulate than Martina Navratilova or Chris Evert, he says, also forget that these two greats struggled in front of microphones earlier in their careers. What most surprised Feinstein about the players? "How much these guys give up," he answers. Many attended special tennis schools from an early age and turned pro at 17 - or younger. "Everything goes out the window," Feinstein says. "There's no normalcy. They don't go to high school like you and I went to high school.... [Defending United States Open champion] Pete Sampras never had a date until a year ago. I'm not sure that the payback is so great that its worth it." Feinstein expects to see a "supertour" evolve soon in both men's and women's tennis, forcing top players to meet more often. Because of conflicting events and the lure of lucrative exhibition matches, some high-profile players rarely meet outside the four "Grand Slam" events: the French Open, Wimbledon, the US Open, and the Australian Open. What's missing today, he says, are classic showdowns. "I don't think it's a coincidence that tennis was at its peak of popularity in the early '80s when the Evert-Navratilova rivalry was maturing on the women's tour, when the men's tour had [Bjorn] Borg and [Jimmy] Connors, Borg and McEnroe, McEnroe and Connors.... Let's face it: When those people were playing, if you had any interest in tennis, you wanted to watch." Among today's top players, Boris Becker and Stefan Edberg "have played 28 times. I don't think anybody has been captured by their rivalry." You need magnetic stars, Feinstein says. "I think it was great for golf when this guy John Daly won the PGA the way he did. You had people who don't follow the sport at all saying, 'Who is this guy?'... Tennis needs this kind of story." Not that he doesn't think highly of Becker, perhaps the best-known Top 10 player. "Boris Becker is one of the brightest athletes I've ever come across," Feinstein says. "He's a guy who really understands there is a certain silliness in the fact that he's a multi-millionaire because he can hit a tennis ball." Among the women he admires are Steffi Graf, who "people don't understand" because she's actually painfully shy, and Zina Garrison, a highly talented black player who has "battled the inherent racism that exists in tennis." Feinstein plans to be at the US Open, which begins Monday. Then he'd like to take on baseball, spending a season with a lesser-known team on the rise, like the Atlanta Braves or the Chicago White Sox. Or maybe the Pittsburgh Pirates, a contender he thinks could be dismantled soon through free agency. If so, the team may end up as "symbolic of baseball in the '90s," he says.