Angling combines contemplation and action, Izaak Walton wrote; in this tale of a trout there is too little of either. Perhaps it is unfair to expect more of a book that is modest and good-natured, but Humphrey is more than a fisherman: He is the author of Home From the Hill and other honorable works of fiction.

The trout weigh 30 pounds, and you can judge for yourself whether that makes this a work of fiction, too; Humphrey urges, us to take it as fact. He discovers the trout in a Berkshire Mountain stream, and spends the better part of a fishing season on his stomach, studying if from the bank. So noble a creature must be caught in the most sporting way possible, he decides and he studies the arcane art of dry-fly fishing as well. On the last day of the season, Humphrey manages to hook the trout:

"Up and up he went until he had risen into the bright sunshine, and there, in defiance of gravity, in suspension of time, he hung. He shook himself down his entire length. The spray that scattered from him caught the light and became a perfect rainbow in miniature. Set in that aureole of his own colors that streamed in bands from him, he gave a final toss of his head, breaking my leader with insolent ease, did a flip, dove and re-entered the water with a splash . . .

His fish gets away, but it is one of the book's best moments, because it is one of the few moments Humphrey bothers to dramatize. Instead of using his novelist's skills, Humphrey more often tells, in his genial, literate voice in the lazy way a story might be told over a campfire, tale that is unremarkable except for the titanic trout.

My Moby Dick serves best as a reminder to turn, or return to the novels and short stories for which its author is known. (Doublday, $6.95)