It may be because he has been in racing for nearly 80 years. It may be because he's one of the rare trainers to take more than one Triple Crown. It may only be because sometime, once, a sportswriter with the knack for it dubbed him with a nickname that stuck because it gave off overtones of what a clear track was like on a beautiful day. Whatever the reasons, Sunny Jim Fitzsimmons has been and at age 87 still is by all odds the best known and most revered figure in American horse racing. His colorful career as supervisor of the working lives of some of the best horses ever to fly the distance began when he was only about five: his family's home stood in the center of the area where a new racetrack was being built. Surrounded always by a large, boisterous Irish family, teamed with a wise and thrifty spouse, fortified by a youthful background of hard work, relentless honesty, and long, long experience with horseflesh, Mr. Fitz took everything in steride. Today, although doubled up with arthritis, he still attends to the work he has always loved. His biography is pungent, nostalgic, and very much alive, a fine tribute with a lot of information on the history of horse racing in this country. Halftones.