At a Magic Threshold : POSSESSION: A Romance <i> By A. S. Byatt (Random House: $22.95; 546 pp.) </i>

Three-thirty in the morning, London, September, 1986. Roland Michell, a part-time research assistant to Prof. Blackadder (who since 1951 has been editing the complete works of 19th-Century poet Randolph Henry Ash), checks out Ash’s own copy of Vico’s “Principi di una Scienza Nuova.� It looks as if no one has touched this book--well, maybe--since Ash himself last touched it.

Roland comes from the urban lower-middle class. He should have used his university education to get a job. But in Thatcher’s England there aren’t any. Roland’s dispirited girlfriend, Val, hasn’t even passed her exams. After her brief fling with English Lit, she types, now, for a living. The pair have ceased to love each other. Still, Roland derives a quiet pleasure just being here in the library, on an ordinary morning: “Here Carlyle had come, here George Eliot had progressed through the bookshelves. . . . Here Randolph Henry Ash had come, cramming his elastic mind and memory with unconsidered trifles from History and Topography . . .�

So Roland, a failure as a lover and a failure in his field, opens Ash’s copy of Vico. Finds some notes and scraps. And by now, still on Page 5 of this 546-page book, the reader of “Possession� may be wondering: Why Ash ? Isn’t he an all-too-obvious evocation of Robert Browning, with those tiresome dramatic monologues of his, and hasn’t the point already been made that Ash lived a long, respectable, essentially uninteresting life?

But Roland turns a page. And then another. And finds not one, but two drafts of what could almost be a love letter, written by Ash, full of revisions and cross-outs and a terrible “urgency.� Roland palms the letters! From then on, his life becomes electric, crammed with meaning. He is “word-obsessed.� He has become, in the first sense of this novel’s title, “possessed.�

Now is the time to lament the perhaps insignificant fact that there is no feminine word for “masterpiece.� Because A. S. Byatt, student and biographer of Iris Murdoch, explicator of Wordsworth and Coleridge, has crafted in this just-announced Booker Prize winner a masterpiece of wordplay and adventure, a sampler of styles, a quilt of scholarly methods and a novel that compares with both Stendahl and Joyce: Characters race to their destinies, but stop, on the way, to rejoice in language in all its ramifications. The history of English literature is encompassed; the state of scholarship is brutally examined (and found wanting), but the world itself is praised, and the humans who quest--as in any real romance--after knowledge swim in adventure and drown, ultimately, in happy endings.

Roland takes his letters home. Val sneers. They are nothing in the real world. She has seen the real world and it is a dreadful place. The next day, Roland, researching, finds that Ash has met a certain Christabel LaMotte at a scholars’ breakfast. (For Christabel LaMotte, read Christina Rossetti.) Soon, Roland encounters an academic rival, the vulpine Fergus Wolff, who is engaged in deconstruction, and has met--at a “Conference on Sexuality and Textuality�--one of the two living experts on Christabel, a Dr. Maud Bailey, who teaches at another university.

Remember, when the author calls this narrative a “romance,� she doesn’t mean a bodice-ripper. Byatt is taking all that material that originally came from Brittany, the place in our literature where the natural and the supernatural embrace, at a magic threshold, as it were, where humans may meet--or turn into--angels or devils or goblins. It may not be absolutely necessary to remember that the medieval “Roland� was Charlemagne’s sidekick and a hero of France; that “Maud� was made immortal by Alfred, Lord Tennyson; that with a boss named Blackadder, you could be in some trouble.

Maud is as chilly as she is beautiful. But when she hears that her Christabel may have had something to do with Roland’s Ash, she too becomes possessed. She is related distantly to Christabel’s family, and she and Roland set out, in rain and gloomy weather, to a decaying castle where . . . they find the correspondence! Letters that could change the history of a nice chunk of English literature. For if the boring Randolph Henry Ash is the “possession,� by now, of the stolid middle-of-the-road white male academy, Christabel LaMotte, with her frail, morbid poems and her mordant tales for children, has become the property of militant feminists, who are busily building a myth around her and her friend--perhaps lover--Blanche Glover, who lived together in a mild perversion of Coventry Patmore’s idealized domesticity. These two “Angels in the House� made jams and jellies--and probably made love.

So in the real world, as Val (only she really is going to turn out to be Valiant) reminds us, people are poor and people are dying, and a few scribbled lines mean “nothing� in the scheme of things. But in the really real world of the imagination, and the pure search for knowledge, the correspondence of Randolph Henry Ash and Christabel LaMotte, who did love, means everything. It means the whole world. And everyone in this small academic world wants part of it.

Fergus Wolff wants to further his career with these letters. As a byproduct, he’d like to “deconstruct� Maud, for the mean hell of it. Blackadder, his life lived in the British Museum basement (in total subservience to the memory of Ash), is frantic when he hears of the letters, and of Roland and Maud’s blatant thievery.

And over from America come two particularly peculiar American specimens. Mortimer Cropper, who believes that to possess the object is to possess poetic magic, has bought manuscripts and letters and the poet’s watch, as if that mechanism could give him a heart. Prof. Leonora Stearn pens ferocious and absolutely unreadable feminist criticism, but in person she’s even more scary--absolutely determined to sleep with every man and woman she comes up against.

Will Roland and Maud come into possession of the letters, and of their own rightful lives? Will they ever get to fall in love? (Unlike their 19th-Century predecessors, they must distrust love, especially romantic love, as a faulty ideological construct.) We’re given hundreds of pages to find out. Many of these pages are taken up with Ash’s long poems, and LaMotte’s short ones. Or LaMotte’s fables, which really aren’t for children. (There’s as much Tennyson as Browning in this Ash, and as much Emily Dickinson as Rossetti in Christabel.) You learn to love these poets who love so tragically.

And you read with fascinated horror the self-serving blather of Mortimer Cropper and are bored stiff with Leonora’s turgid feminist rant. And then, just because she’s showing off, Byatt throws in the 19th-Century journal of a precocious adolescent in Brittany. You’re encouraged to think about “thresholds�: If artists strive and pine and yearn to live on, through their work, into the future, it’s the scholars, with equal tenderness and yearning, who lean and look, further and further back into the past, willing, finally, to lose themselves in the search for those lost artists. They meet on the threshold where knowledge is.

Byatt believes that all these people should get their happy endings. About 80 pages from the finish, she begins pitching them out: Something for Val. Something for a self-effacing guardian of Ash’s wife’s journal. Something for Blackadder. Maud finds out who she is. Roland, noble knight, finds the real object of his quest. (And who said there could only be one?)

You sail on the wind in this book. You give yourself up to it. And it rewards you a thousand times.