Tibet: behind the screen of myth and legend; Tibet and Its History, by Hugh M. Richardson. Boulder, Colo.: Shambhala. 327 pp. $19.95.

History, it has been said, is often nothing more than ''retrospective prophecy,'' which seems a particularly apt way to define both the immediate and lasting impact of Hugh Richardson's ''Tibet and Its History.'' Systematically, as if substantiating a scientific theorem, the author identifies those events and lapses in judgment that ultimately brought about Tibet's annexation by China and the brutal subjugation of the people of this remote Himalayan nation.

Originally published in 1962, but here amended and updated to include more recent information on Tibet since the Chinese occupation in 1950, the book is a comprehensive and thoughtful summation of more than 1,500 years of Tibetan civilizations.

Existing throughout much of its history in a splendid self-imposed exile, Tibet was shielded from the outside world by the towering Himalayas and the desolate salt pans of the western Gobi desert area. In their isolation, the Tibetan people were able to build a theocracy of medieval power and splendor that was to endure for close to 500 years, until the advance guard of the Chinese People's Liberation Army entered Lhasa just five months after the defeat of Chiang Kai-shek and his Nationalist forces.

Tibet's reluctant absorption into the nascent People's Republic of China is presented here as the culmination of a gradual process of Chinese encroachment that began in the mid-17th century. Always wary of their powerful neighbor to the east, the Tibetans did their utmost to placate the vanity of the Ching Dynasty emperors, while retaining for themselves all the broader privileges and prestige of a peaceable nation whose influence extended from the Mongolian grasslands to the bazaars of Samarkand.

Ultimately, it was Tibet's own carefully guarded isolation that left the nation incapable of withstanding the assault on its independence waged by the Chinese. Tibet in 1950 maintained no formal diplomatic relations with any of the world's nations and was seen in most of the Western capitals as little more than a secluded paradise vaguely suggestive of the Shangri-La depicted in James Hilton's ''Lost Horizon.'' As a consequence, when the Chinese Army massed on its northern and eastern borders in the winter of 1950, Tibet was unable to call on the assistance of either its neighbors or those Western nations foresworn to oppose the expansion of communism in Asia.

The Dalai Lama, the spiritual and temporal leader of the world's 6 million Tibetans, fled his homeland after an abortive and ill-fated anti-Chinese uprising in 1959 and took up residence in northern India. Nearly 100,000 Tibetans joined him in exile, making the perilous crossing of the Himalayas just steps ahead of the pursuing Chinese Army.

Ironically, the sad diaspora of the Tibetan people has achieved what more than 15 centuries of Tibetan independence could not: the wide dissemination of informaton about a nation with an incomparably rich philosophical and artistic legacy. From their homeland-in-exile in India, the Tibetan people for the first time have exposed their unique heritage to the scrutiny of Western scholars, a historic unveiling which has helped fuel a dramatic surge in interest in the culture and theology of Tibet.

This book, written with an extremely deft hand by a former British civil servant who was stationed in Lhasa during the 1930s, is a fascinating and engaging account of a people and a culture too long hidden behind an imposing screen of myth and hyperbole.

Shambhala Press, publisher of ''Tibet and Its History,'' has also recently published one of the first books devoted to the study of Tibetan religious painting. Tibetan Thangka Painting was written by two British art historians, David and Janice Jackson, who studied drawing and applique techniques with Tibetan master painters in Nepal and northern India.

The book, however, is less a formal analysis of the purpose and ritual significance of Tibetan religious art than a technical ''how to'' guide with specific advice on sketching proper perspective and mixing earth-tone dyes.

Such an obscure approach to a decidedly esoteric subject is regrettable. We are increasingly dependent on small presses such as Shambhala to publish books of impressive intellectual scope and substance for limited audiences. So it is especially disappointing when one of their efforts in this direction is of such exceedingly narrow appeal as to exclude rather than attract most prospective readers.