'S always 'a pleasure, Mr. Waugh; A Little Learning, by Evelyn Waugh. Boston and Toronto: Little, Brown & Co. 234 pp. $14.95. When the Going was Good, by Evelyn Waugh. Boston and Toronto: Little, Brown & Co. 298 pp. $14.95.

There are at least three decades in the last 100 years - the '90s, the '20s and, more recently, the '60s - which stand out, at least in social terms, like a trio of brilliant and brash macaws let loose in a city square of drab, handout-seeking sparrows and pigeons. The '60s are still too close for dispassionate appraisal, but the '20s have now reached one of those weigh stations where historians, literary critics, and other guardians of culture gather to check out an era.

Of the English writers associated with that period perhaps only D. H. Lawrence and Evelyn Waugh have made that happy transition from contemporary celebrity to major literary figure. While writers like Wyndham Lewis and Aldous Huxley, acclaimed at the time as brilliant and daring, now seem mere literary curiosities, Waugh on the contrary has worn well. The reissue this spring of his one volume of autobiography, ''A Little Learning,'' and ''When the Going Was Good,'' a collection of travel pieces, gives those who were first introduced to him by the television series ''Brideshead Revisited'' an opportunity to make a closer acquaintance.

Evelyn Waugh produced ''A Little Learning'' late in his life when, as he wrote, ''one has lost all curiosity about the future.'' But, though his talents had been acknowledged, he was still driven by that strong moral sense which had always informed his writings and his life, and which gives his work an edge and depth that a more complacent writer could never achieve.

Waugh's childhood, spent in a large house outside London, was ''save for a few pale shadows . . . an even glow of pure happiness.'' He saw little of his elder brother Alec, away at boarding school, or of his father, managing director of a publishing firm, who, for the first seven years of Waugh's life ''was a figure of minor importance and interest'' and one whose ''sedentary and cerebral occupations appeared ignominious to me in my early childhood.'' His mother and Nanny Lucy were ''the sole objects of love in the lustrum between pram and prep school which love permeated and directed.''

The idyllic childhood ended when he went off to Lancing, a boarding school which emphasized High Churchmanship but was otherwise typical of the time. But for Waugh, who had loved his home and the closeness of family life, the change was drastic: ''The exclusion of feminine influence and domestic life was absolute. We never entered a human dwelling or saw a shop; to a boy like myself coming straight from home, the experience was chilling.''

World War I exacerbated matters. Everything ''was of necessity a makeshift - the clothes we wore, the food we ate, the books we worked with, the masters who should have taught us. We were cold, shabby, and hungry in the ethos, not of free Sparta, but of some beleaguered, enervated, and forgotten garrison.''

Conditions improved at the end of the war. Waugh, in the typical adolescent response of a boy who is no good at athletics, became the ''Bolshie,'' the iconoclast who led a small like-minded coterie in malicious and often obnoxious attacks on conventional school idols. He also declared that he was ''an atheist in all except the courage to admit it to himself.''

Lancing was followed by Oxford, and the parallels between the experiences of Brideshead's Charles Ryder and Waugh himself are close, though there was no Sebastian, only a group of brilliant and witty young men who became his friends, who tasted everything that Oxford could offer, and consumed as much as they could hold. But the autobiography ends at one of the lowest points in his life, when he was teaching at a third-rate private school in Wales, in debt, his first novel rejected, his career as an artist a failure. A year later it all changed, but Waugh did not live long enough to write the volumes that would have reflected the dramatic improvement in his fortunes.

''When the Going Was Good'' reflects another phase in Waugh's life. In ''Abroad,'' Paul Fussell has lamented that it is no longer possible to write the kind of travel book so popular in the '20s and '30s. We go abroad to eat and shop, not, as Waugh did, ''because there is a fascination in dim and barbarous places, and particularly in the borderlines of conflicting cultures and states of development . . .''

These pieces are taken from accounts of his travels in the Mediterranean, Brazil, East Africa and Abyssinia. They reflect the discrepancy between the ideal and the realized so fundamental to Waugh. His eye for the absurd and the bizarre is acute. In these savage and out-of-the-way places people were not that different from the sophisticates in London. On returning to London from Africa he notes:

''I was back in the center of the Empire, and in the spot where, at the moment, 'everyone' was going. Next day the gossip-writers would chronicle who was assembled in that rowdy cellar, hotter than Zanzibar, noisier than the market at Harar, more reckless of the decencies of hospitality than the taverns of Kabalo or Tabora. And a month later the wives of English officials would read about it, and stare out across the bush or jungle or desert or forest or golf links, and envy their sisters at home, and wish they had the money to marry rich men.''

There are a few heroes in these out-of-the-way places: his Armenian driver in Abyssinia; Bain, the British official in Guiana; and the German hotel keeper in Addis Ababa. But few others are as admirable. Most are colorful, mendacious rogues, human flotsam washed up in lonely and uncomfortable corners of the world. Waugh was primarily a novelist who noted the infinite variety of man rather than of nature. There is his marvelous bathetic description of Etna at sunset: ''. . . the mountain almost invisible in a blur of pastel gray, flowing on the top and then repeating its shape, as though reflected, in a wisp of gray smoke, with the whole horizon behind radiant with pink light, fading gently into a gray pastel sky. Nothing I have ever seen in art or nature was quite so revolting.''

It is a pleasure to reread these pieces. Waugh writes with a consummate grace , wit, and humor that is as fresh as when the pieces first appeared. They endure because he infused them with preoccupations of a wider, more lasting kind than mere diversion and amusement.