The Hoard of the Himalayas, by Larry Healey. New York: Dodd, Mead & Co. 190 pp. $7.95.

Climbing the High Himalayas is the dream of many an experienced mountaineer. For Jimmy Dalton, this book's fictional hero, a rugged 15-year-old American, it becomes a reality. But Jimmy's eight-person expedition has set out not only to scale the world's highest peaks, but also to find the ''Senti Devi'' (''Shy Goddess''), a peak considered ''the greatest unexplored mystery left on earth.''

When Jimmy arrives in India, a mysterious stranger offers him $5 million to find the Casket of Amphetrios, containing a flawless ruby larger than a man's fist. The stranger tells Jimmy that one member of the expedition is also more interested in the legendary casket, hidden in the depth of the Senti Devi, than conquering its summit.

As they start their climb, mysterious accidents and unexplained deaths plague the climbers. But the culprit isn't uncovered until the group reaches the Senti Devi and begins the descent from the mountain.

Healey's insights into a teen-ager's thinking make his winning portrayal of Jimmy's relationship with the adults in the expedition possible.