WITH THE popularity of English upper-class humor of the 1920s still growing, it's hard to imagine why A. P. Herbert's Topsy books have not already been rediscovered, republished and re-lionized. Herbert's heroine, Topsy Trout, was the epitome of the high-born British Modern Girl, whose letters to her friends Trix were a regular feature of Punch during the '20s. She was as quintessentially British as her counterpart across the Atlantic, Lorelei Lee, was American, and like Lorelei she brought the withering light of a "fresh mind" to the social absurdities of the decade. In 1928 and 1929, Herbert collected the pieces into books which, when read now, are souvenirs of their era and which retain their brightness and humor as successfully as do E.F. Benson's Lucia novels.

Topsy's strong point is her style. She pushes archness to its limits, italicizing without restraint to make sure the point is made, and is not too worried about spelling. In writing about her books, it is impossible to resist the temptation to quote at length. Here she is describing someone she would have preferred not to have met:

" . . . Well I forget if I've mentioned a rather septic Baronet I met at the Antons, some time ago, my dear too festering, well he's a fiancier or a shareholder or something and one mass of heartiness, my dear shouts at you at breakfast when a girl's not conscious, and also he's what I call a chronic handler, you know my dear he simply can't keep his hands from pawing and slapping, well my dear by the end of the week-end every man in the party is black and blue with friendly buffets and they say once at a golf-club he split an Alderman's liver out of sheer cammaraderie . . . "

Each of her letters to Trix turns out to be an account, as full of reversals and vicissitudes as The Odyssey, of a social ordeal she has endured after being brought into collision with some fad (weight reduction at the Turkish bath), moral attitudes (the pursuit of ideals), or current issues (fundamentalism vs. evolution). While visiting Uncle Arthur and Aunt Margaret, for example, she finds that the only thing that arouses as much enthusiasm as idealism is hunting. "There's the most proper young Guardee officer whose one idea is to avoid anything vulgar," she writes, "and my dear they spend the whole day killing things, well Aunt Margaret doesn't kill anything much except slugs and snails, but she kills them in the most barbarous manner, my dear if you could see her creeping round the garden with a bag of salt and watching them shrivel . . . "

High culture also routinely gets a going over. In one especially funny letter Topsy tells of a reviewing assignment given to her by the editor of Undies. She is supposed to take her "fresh mind" to Hammersmith to write up a production of some play called Othello. After sitting through it she admits that she's rarely seen such trash, and gives her reasons why:

" . . . There's an obstruse villain called Yahgo or something who never stops lying and my dear for no reason at all that I could discover . . . well he keeps telling the old black man that the white girl has a fancy-friend, well my dear they've only been married about ten days but the black man merely laps it up, one moment he's Nature's honeymooner and the next he's knocking her down, and what I thought was so perfectly heterodox he was supposed to be the world's successful general but my dear I've always understood the sole point of a real he-soldier is that they're the most elaborate judges of character and always know when you're lying, and if this black man couldn't see through Yahgo it's too unsatisfying thinking about him winning a single battle against the Turks . . . " BEHIND ALL the lampooning and eyebrow raising that make up the bulk of Topsy's letters there is a continuity that yields a kind of plot. Generally, Topsy is squired from disaster to disaster by an admirer, Mr. Albert Haddock, whom she eventually marries. At the time of their wedding, Haddock is campaigning for a seat in Parliament, and as The Trials of Topsy concludes, Haddock is forced to step down because of complications, but Topsy, who has become immensely popular while helping him with his campaign, is easily elected to the seat, which she intends to "keep warm" until Haddock is in a position to run again. Her adventures are carried further in a second volume, Topsy, M.P., which is more purely political in content, but which ends with Topsy giving birth to twins, "as like as two electrons."

Topsy's original readers must have been quite broad-minded, as some of the attitudes Herbert satirizes through Topsy's pronouncements were still no laughing matter in many influential quarters. It's sometimes difficult to tell just how far Topsy has her tongue in her cheek when she takes the most conservative possible stand on a hot issue. When Haddock becomes ill with the flu during his campaign, Topsy ghosts his election address for him. Her speech on foreign policy is:

"FOREIGN AFFAIRS. I'm too saturated with foreign affairs, my dears I don't want to hear a single word about the hairy Lithuanians or the scrofulous Croats or the Poles or the Yaks or any of these redundant Europeans, because of course they're not white men, they don't care two hoots for us, I don't care half a hoot for them, they're miles away, they don't shave, and why we should waste ten minutes oozing about in their malarial affairs I merely can't imagine so what I've always said is Let them wallow, because why we should be the scullery-maid of Europe when meanwhile we've got the most disarming clean-limbed Empire utterly far-flung and starving for a matey glance, and if you ask me my Foreign Policy is eliminate the Foreign Office, because my dears merely all they do is to make jobs for themselves and trouble for us."

The controversy centered on Darwinism gets a grilling, too, with Topsy writing, "If it should turn out that the origins of Man were monkeys one simply couldn't go on, you do see that don't you darling," and then going on to make elaborate fun of the whole issue.

All of this has dated in just the right way. The skepticism and irony with which Herbert, through Topsy, approaches topics that were once solemn prevent the book from fading even at a time when much of its contents belongs to the realm of social history. Most important, Topsy herself is still completely alive; were she revived now she would know just what she thought of everything and probably express it better than most of us. Reading her letters now, it is difficult to believe that they have been out of print for half-a-century -- far too long. It is time for some enterprising publisher to get them back into the bookshops. Bob Halliday writes frequently for Book World on art and literature. Both "The Trials of Topsy" and "Topsy, M.P." are long out-of-print. Copies occasionally may be found in second-hand bookstores and in public libraries.