MARIAN King has no use for those who either boast about or lament their age. Her entry in Who's Who includes no date of birth, and the Washington-born writer pleasantly but firmly declines to provide it to the interviewer.

The timeless island on which she lives, however, does display one calendar -- the procession of her books, marching one by one across her bookcase. That recorder of the unfolding years has now reached and passed the half-century mark. The ABC Game Book , by Marian King, was published in 1928; Adventures in Art , by Marian King, her 29th and most recent book, was published last fall.

Marian King's books don't deal in the controversy and contention with which much of her city concerns itself. Often, as is the case with her current volume, they go on sale unnoticed by reviewers. Usually they are aimed at children. But as King leans back reflectively in an armchair in the sun-filled Connecticut Avenue apartment where she lives and works, she betrays the same perturbed, faintly worried, almost obsessive preoccupation with words that is the writer's blessing and curse. Indeed, writing finally drove her away from her second love, playing tournament tennis.

"It's not an easy job," she says of the craft. "You're with it all the time. I know that when I was playing in a national tournament one time in Boston, I was playing with Hazel Whiteman -- the Whiteman Cup, she took me on -- and I was writing my book Young Mary Stuart .

"I was up at the net and ball was just about an inch away from my racket -- and I missed it. She came up behind me, scooped it up and put it in place. And all she said to me was, 'Put that period on that sentence, and play tennis!'

"The thing is, if you're writing a book, whatever you're doing in the writing field, at least with me, it never leaves me until it's done. I work with it even sleeping sometimes.

"After that I decided I'd better stay with my writing and not play tournament tennis, just play for fun...."

The thwuck... thwuck... thwuck of the well-rallied tennis ball has supplied the background punctuation to many of King's doings.

"I've just loved athletics all my life," she says. "When I was a child I used to get out and play baseball with the boys. As I grew older I could outdo them."

Finally one day her uncle Philip King, who with her father Joseph and another uncle, Harry, operated the long-gone King's Palace department store on 7th Street, taught her to play tennis. Philip King had been a football player at Princeton; he took the little girl out on the court after she lamented that girls weren't given the opportunity of the gridiron. The lessons took: "I always loved to hit the ball," says King.

It was at a tennis court that The ABC of Games , King's first book, had its origin. As she tells the story: "I was playing in a tennis tournament, an indoor tournament, and the Davis Cup captain was on the sideline. I was standing next to him, and he said, 'Oh, Marian, leths do a book." And I said, 'Yes, let's do it.'"

Next day the tennis captain hadn't come up with his book idea, but King had her title and the basic format of the little book she then sat down to write, illustrate -- and see published.

King has a dispassionate, almost uninterested regard for her completed books. "When a book is published, to be perfectly frank, I hardly ever reread it," she says. It is a sentiment echoed in a note she received one time from the poet Marianne Moore, a note King had framed and hung on her living room wall:

"Dear Marian King,

"Thank you for being interested in the award. As for my C. Poems, on no account buy it -- mostly in former book and a library has more room for books than we have.

"MMoore Aug 28, 1967"

Moore and King shared more than an interest in writing. King chuckles as she recalls an evening when they were having dinner together at a Longchamps restaurant in New York. Gradually the two became aware that a man at a nearby table was listening in on their conversation. Finally he rose and came over to their table. "Forgive me for eavesdropping," he said, "but I have never before heard two women discussing sports so intelligently."

For King, watching an athletic event, especially tennis, is an aesthetic experience. "I love to watch tennis," she says. "When it's played well, it's beautiful. It's rhythmic and it has a great deal of grace."

The attentive eye that she first learned to focus on the ball provides a distinctive tone to the commentaries on paintings in the National Gallery of Art that made up Adventures in Art . Rather than concerning herself with the artist's technique, she tends to concentrate on what she sees and imagines about the content of the painting. Mary Cassatt's "Child in a Straw Hat," a painting to which King often refers, offers a characteristic example:

"The child's dark brown eyes look out of the picture, probably at the artist painting her," King writes. "She is not an eager model, for her lips are pursed in a pout. Her casually crossed hands suggest her patience as she poses for her portrait.She would rather, no doubt, be playing in the summer sun with other children."

King recalls discussing the Cassatt painting one day with a group of schoolchildren. "One little girl said to me, 'Miss King, were you that little girl when you were little?' I said, 'No, I whish I were -- she's so cute and pouty, and I guess I pouted too.'" King pauses, then adds reflectively, "Children will pick out things sometimes that you won't see."

Only once in all the years has King failed to see a book through to publication. That was a children's novel about a Czech refugee. "I made the characters too good," she confesses. "I didn't give them enough of the opposite side, where they'd be mean or nasty. I just couldn't put it in.

"So I guess I'm not that kind of writer."

No, Marian King is not that king of writer. In her apartment, it is as if the sun always shines, and time passes only to bring new joys. It is as if her life were fully attuned to the principle once laid down by the French philosopher Henri Bergson: "To exist is to change, to change is to mature, to mature is to go on creating oneself endlessly.