BY AMERICAN RULES Rebecca West has been a senior citizen for almost 30 years. During that time she has written her best novels, The Fountain Overflows and The Birds Fall Down, and some of her most important nonfiction. In the last year or so, she has given a lengthy interview to The Paris Review, filled an installment of Bill Moyers' Journal with her sparkling reminiscences, and served as the most ebullient witness in Warren Beatty's Reds. She reviews books regularly for a London newspaper; her autobiography and a novel are in progress. Now comes a spate of her books, one of them brand new. All this invaluable activity by a writer who turns 90 at the end of the year underscores the ultimate futility of categorization by age.

But then Rebecca West has always been label-proof. At the outset of her career, she dropped her given name, Cicily Fairfield, and took a pseudonym (from the nonconformist heroine of Ibsen's Rosmersholm). She is a lifelong feminist, a quondam socialist. She had an affair with--and a child by--H.G. Wells. If all this forms a pattern of rebellion, she broke it by marrying a banker in 1930 and living contentedly with him until his death in 1968. She became a certified doyene of English letters on being designated a Dame Commander, Order of the British Empire, in 1959.

Her work is equally hard to categorize. She has written fiction, reportage, criticism, the essay, biography, history, travelogue, and satire, but often she has transformed these genres to suit her purposes. She was one of the first literary figures to take up reporting after making a name for herself as a novelist. The result was a series of expansive, intuitive reports for The New Yorker in the '40s and '50s--on murder, treason, and the Nuremberg Trials--that was a precursor of both the exuberant New Journalism practiced by Norman Mailer and Tom Wolfe and the neoclassical literature of fact practiced by John McPhee.

And what are the classifiers to make of Black Lamb and Grey Falcon, her masterpiece and one of the century's most enriching books? This 1,200-page work about Yugoslavia is a tapestry woven of travelogue, history, philosphy, autobiography, and anthropology that beggars the Dewey Decimal System. Rebecca West once advised a grad student against doing a thesis on her because her writings "could not fuse to make a picture of a writer, since the interstices were too wide."

But, of course, Dame Rebecca's writings comprise several common topics and themes, such as the oppression of women, espionage, crimes of state, and the stupidity of the death wish. Her cantering prose style, by turns gnomic and sensuous, is another unifying element. Both qualities are evident in the opening of her dispatch from Nuremberg in 1946: "There rushed up towards the plane the astonishing face of the world's enemy: pine woods on little hills, grey-green glossy lakes, too small ever to be anything but smooth, gardens tall with red- tongued beans, fields striped with copper wheat, russet- roofed villages with headlong gables and pumpkin-steeple churches that no architect over seven could have designed."

And in any event, the new book, 1900, shows what an asset wide interstices can be. It's a freewheeling assemblage of memoir, history, and essay that moves backward and forward from a benchmark year. It's also a showcase for West's effortless wit, as seen in this literary snapshot of those magisterial artists, Henry James and John Singer Sargent. "Though Great Britain was their chosen field and their special inspiration, they were both of old American stock, and both, strangely enough, looked like English butlers. They were massive and thick-set rather than lordly men, with hands and arms indicating that they could polish silver against anyone."

And there is an irreverent vignette of Queen Victoria in her last years that begins, "I have very early memories of her as nothing more than a squat bundle of black clothes, mourning clothes (she went on and on mourning Prince Albert like a dog that will not give up a bone)." The piquant text is embellished with remarkable photographs and paintings: West and her picture researcher, Faith Perkins, were clever enough to forage outside the same old archives that everyone else consults. Altogether, 1900 may be the brainiest coffee-table book ever published.

The Dial Press has been reprinting notable books by or about women as Virago Modern Classics, and two of Rebecca West's early novels are the latest in the series. The Return of the Soldier (1918) is a short work about a soldier who is furloughed home suffering from shell- shock and amnesia. The dilemma for the three women in his life--his wife (whom he can't remember), his sister, and his first love--is whether to jolt him back to the present or leave him confused; for if they cure him, he will almost certainly be sent back to the front. The novel is a strong blend of touching drama, keen psychology, and evocative nature descriptions.

The eponymous heroine of Harriet Hume (1929) has the odd faculty of reading the mind of a single person, Arnold Condorex, the man she loves. He returns her love but marries someone else to advance his political career. The novel consists of a handful of rendezvous down through the years, in which Harriet repeatedly tries to warn Arnold away from peculation and ruin. There are good things in Harriet Hume, such as Arnold's resentment of the serene way in which Harriet lays bare his corrupt plans. " 'Oh, if she would only speak too loud,' he thought, 'so that I might tell her not to shout!' " But the narrative tone is excessively arch, the prose cloyingly ornate. For me this is the lone failure among Rebecca West's 20-odd books.

Edited by Jane Marcus, The Young Rebecca samples from the turbulent beginning of a career. Rebecca West was assaulted several times while working to get women the vote, and this collection of early reviews and polemics is a withering indictment of England's male ruling class. At times she can hardly contain her rage. In a speech, the Liberal anti-suffragist Lewis Harcourt had complained of an attempt "to burn my house--or rather the children's wing." Rebecca West replied, "Long may the little Harcourts remain unburned; but there is really no more reason that they should not be burned to death than that (the suffragists) Cecilia Haig and Mary Clarke should have been kicked to death outside St. Stephen's three years ago."

Yet for all its honest anger, The Young Rebecca has moments of quiet celebration, such as this handsome compliment paid Harley Granville-Barker, the critic and dramatist: "Thought bubbles from him like laughter from a healthy child." That was written in 1912. No one fits the description better today than Rebecca West herself.