Aloof, mysterious Tibet as it looked in 1942; A Portrait of Lost Tibet, text by Rosemary Jones Tung, photographs by Ilya Tolstoy and Brooke Dolan. New York: Holt, Rinehart & Winston. $19.95.

Though Tibet is hardly your American Express-card, package-tour deal yet, its Chinese rulers have substantially relaxed their 20-year-old ban on visitors. Since the Chinese annexed Tibet in 1959, toppling its ancient religious power structure, they have kept it aloof from any prying Western, or indeed foreign, eyes. But in recent months a string of travelers have made the trek to that remote mountain land to report what they can of what has happened there.

Though the topography remains the same, there must be extraordinary changes from the society depicted in this remarkable book. It contains a photographic record, in black and white, alas, of a 10-month trek across Tibet in 1942 by two American OSS officers on a diplomatic mission for the Allies. One of them was Leo Tolstoy's grandson.

Even then, Tibet was aloof and remote, a mysterious Shangri-La. Tolstoy and his colleague, Brooke Dolan, did us all a great service by recording it on film. Soon after, Tibet, and many of its traditions, fell to the Chinese Communists. Hence the title "Lost Tibet."

Now Rosemary Jones Tung, an expert on Tibet, in an accompanying text describes the Tibetan society that Tolstoy and Dolan explored. It is probably the record of a society gone for ever, and thus an immensely valuable volume.