BERYL BAINBRIDGE, author of Sweet William, Injury Time and Young Adof [all published here in hardcover by Geroge Braziller] , is a woman of 44, slim and energetic, her long black bangs streaked with gray. Her most striking fearture is her large, expressive, dark gray eyes; she looks right at you when she is a name-an established, recognizable writer whose books sell very well. Here, she is less well known; her readership is relatively small, but growing. No longer married, she has three children-a son, 22, and two daughters, 20 and 14. In the last seven years she has published seven novels. Why does she feel compelled to, in her own words, "keep up every year doing one?"

"It was my publisher's idea to begin with and i suppose i've got into the habit of it now. i'd get worried if i'd ever do one again. This is the first year i haven't begun one in January or February." Starting in her childhood, she followed a stage career, first as a tap-dancer, then later in radio and rep ertory theatre. "But i've always written."

Has Bainbridge ever considered non-fiction? "No, i really haven't got the education for that sort of thing. The bit of what i laughingly call research that i did on Young Adolf i quite enjoyed. I felt rather educated, rushing around looking in libraries. The facts are that Adolf Hitler's half-brother Alois owned a restaurant in Liverpool as early as 1910. He married an Irish Liverpool girl called Bridget Dowling. They had a son called William Patrick Hitler. That's fact. He's here now , in the States. In 1941, through some amazing ridiculousness, Britain's war effort; she worked for the Red Cross in Washington. William Patrick went lecturing at American universities on 'My Uncle Adolf.' Obviously, they'd thought they could cash in on their relationship with Hitler, so they went first to Germany, but they were a terrible embarrassment to Uncle Adolf. By this time they'd already caused trouble at home [Britian] by claiming their relationship to Der Fuehrer, so they weren't really welcom anywhere. And so they came here. In 1941, Bridget wrote her memoirs, and the first four pages say that in 1912 she and Alois went to meet Alois' sister Angela, arriving from Linz, and off the train instead came Adlf. I read the memoirs pretty thoroughly, because i'm doing a two-part play for the BBC based on the memoirs. The part of them that seems the most real is the part about Adolf coming to Liverpool. It's the most understated, whether it's true or not. There's no proof that he came, but there's no proof that he didn't."

Why did Bainbridge choose the 23-year-old Adolf Hitler to write about in the first place? Did it have to do with Liverpool? "Oh, yes, i come from Liverpool, and i have written about it before. It also had a lot to do with my father, who had the mostappalling rages. He'd jump up and down and gibber, I was going to base Adolf on my father, but it became too involved. Adolf became too real, too rounded a character and i had to begin the book again."

Did she feel sympathy for her "hero"? "Yes. . . n. . . i have to be careful about that. If he'd not had the name Adolf Hitler, one might have had sympathy for him. But i was very careful to write it, especially at the beginning, through the eyes of the other characters. It was only towards the end that you got to know what Adolf, eas thinking, because he was on his own in those scenes, and i had to do it from his point of view. But i've got a disposition to always see normality in oddity. Certainly, had Adolf been around now, and applied to a colege of art in England, he'd have been in like that. So you had a persecuted lad who lost his mother to cancer; and his whole thing was that hehd been done out of art school. He felt very frustrated as an artist. In that sense, one can see why he was so odd. Actually, he was no more anti-Semitic than any other German, because anti-Semitism was absolutely taught in the schools and the universities. And he wasn't particularly interested in politics, but the war gave him a family unit for the first time-the army -although he didn't mix very well. He won the Iron Cross, andthat gave him a sense of security. Then the war ended, and he wass nobody again. He just wandered in out of the cold, into a meeting in Munich, and somebbody thought, by God! that boy can speak, and dragged him in, and he was away! And it was as random as that. You read all the time about famous people who groomed themselves from the age of 15, but Adolf never did that. Never! He went in every other direction you could think of."

What is Bainbridge working on now, apasrt from the two-part BBC play? She plans a theatrical play of Injury Time (just out, in paperback, from Bantam). She's also thinking of doing a sequel to Injury Time , set in Corfu, where she goes every year. "I've got an idea in my head, about what Injury Time was originally meant to be. My mother was an almost flighty lady who liked life and running around, but who was highly respectable. She did everything because the neighbors were watching, and in her lifetime all the rules went out the window. She was a bit bitter about that. I rebelled very early on, which annoyed my mother no end, because she was waiting to see my fall. And it didn't come. I'd like to write about that." CAPTION: Picture, Beryl Bainbridge, by Jerry Bauer