To be asked to pitch for the Mets was like inviting Helen Hayes to appear on Hee Haw, or asking Leonard Bernstein to conduct The Rolling Stones,"" Mr. Sullivan says, reflecting on Tom Seaver's draft. And he backs himself up with a breezy parody of the Mets' bitter genesis and slap-hapless history which took a turn for the less worse when Seaver came along -- the ""imperturbable,"" supremely ""confident,"" ""mature,"" cerebral right-hander from Fresno, California. By way of Fairbanks, Alaska, where he summered with the amateur Goldpanners, already evincing the ""poise"" that would earn him the Rookie of the Year Award and a place on the All-Star Team after his first New York season. A pro who can really ""finesse a hitter,"" Tom bears some uncanny resemblances to Christy (Matty) Mathewson, and a whole digressive but liberally lore-studded chapter parallels their pitching and life styles. Sullivan quotes from the quotable all the way through and replays Seaver's critical games, winding up with the '69 Series and the extra-curricular manifestations of the determination and discipline that make Tom so much the model of a modern Major Leaguer.