Pick of the paperbacks; A Short Walk in the Hindu Kush, by Eric Newby. New York: Penguin Books. $4.95.

Those who enjoy British understatement and humor will enjoy Eric Newby's tale of how he and his companion undertook a mountaineering exploit high up in the rocky wilderness of northeastern Afghanistan.

Nuristan was their goal -- a remote, almost impenetrable, corner of Afghanistan. It was indeed a feat of endurance, which Newby relates in an amusing self-disparaging manner. Some of the situations he describes would be out of place as topics for polite dinner-table conversation. But in their context, they are at least understandable.

In his more serious moments the author gives us some beautifully written passages describing the valleys and gor ges of the Hindu Kush.