MITTS by William Curran (Morrow: $15.95). Ninety-nine percent of baseball fans are concerned chiefly with hitting and pitching, but Curran--a baseball fanatic since the '30s--maintains the real beauty of the game is in fielding, hence the title of this somewhat slight (228 pages) but surprisingly affectionate and entertaining series of tales. A member of the Society for American Baseball Research, Curran comes across like the windy old-timers that you meet at a ballgame--the ones who argue that players used to be so much better than today's high-paid stars. The switch is that Curran will have none of that nostalgia. He insists that today's fielders are far superior to the old-timers. In the seven-game championship playoff in 1985, Curran notes good-naturedly that the two teams made 100 errors between them. One team made 17 errors in the seventh game alone. In discussing fielding by position, Curran nominates the best and the worst players, ending with an all-time roster that includes Johnny Bench, Willie Mays and Brooks Robinson. Any baseball fan knows that a windy old-timer can be a delight or a bore, but Curran is definitely a delight.