My longtime friend Nick Lyons defies easy description. Sometime English professor, sometime writer, full-time book publisher, he is simply this country's foremost man of flyfishing letters. His best book? Spring Creek (169 pp.; 1992. Illustrated. Published by The Atlantic Monthly Press, Dept. FS, 19 Union Sq. W., New York, NY 10003. Hardcover, $20), which is a whole book from start to finish. His other-and equally popular-books are collected flyfishing columns that likewise deserve your attention.