The unpredictable delights of an author's fan mail

One of the joys of having published a first book - besides knowing that you will never again have to go through that particular gestation - is receiving fan mail.

And one of the delights of fan mail is its sheer unpredictability.

I bask in praise from readers who have spent time in the Soviet Union and say that my work has given them insights into their own experience. I resign myself to the opposite in brickbats from those who think I'm too hard - or too soft - on the Russians.

But I especially honor the cris de coeur. A physicist in fusion energy research wrote feelingly about ''the paramount need to continue energetic efforts on arms control until we succeed.'' A small business man, beset by nuclear terror, wrote of his ''anxiety for our five grown children, my country, the world,'' and signaled his commitment to do his mite for mutual understanding by studying Russian at his local university.

All of these responses are familiar to me by now. But today - some two years after publication - there was a new twist. An appeal arrived in the mail saying:

''I read with great interest your book 'From the Yaroslavsky Station.' I have been able to find meanings for every new word I jotted down except this one: 'dystopia.' ''

The letter writer, discreetly identifying herself as an assistant vice-president of a bank by penning her message on the back of her personal memo pad, enclosed a piece of paper for the return definition.

This sort of question I can cope with far more easily than the kind that erupts in random call-in talk shows. In that genre I fumbled most obviously, I suppose, in answering the woman with the soft Tennessee drawl.

''Liz-beth,'' she began, ''mah question is different from everybody else's. Ah heard you mention a 'simovar,' and ah have two that were made in Russia years and years ago; they're antiques, and they're just beautiful when they're polished and shaahned.'' Could I tell her, she wondered, what the samovar on the train was made of?

The real croppers one tries to forget - like the time the disc jockey interviewed me on British armed forces radio. He suddenly asked me what kind of humor Russians like and invited me to tell a Russian joke.

Unfortunately, as I flipped through my memory bank, I couldn't think of a single anecdote that wasn't either obscure, interminable, or scatological. I settled instead for a crisp Polish joke with a Russian angle, then waited for the appreciative guffaw, or at least chuckle.

None came. The studio filled with an awful silence - followed by a deep bass rebuke: ''Well, excuse me, but I don't think that's very funny.''

On the mercilessly live mike I desperately tried to explain the punch line, and just dug myself in deeper and deeper. When the time at last ran out, I fled to the control room to retrieve my coat before slinking away. ''You know,'' confided the technician to me, shaking his head, ''I didn't get that joke, either.''

But the clunkers are offset by the serendipities. Like the London radio interviewer who had just been reading his grandmother's diaries from the time when she commuted seasonally on the Trans-Siberian Railroad between her parents' home at the British legation in Peking and her English boarding school.

We finished our recording session, then lingered in the corridor to exchange tales - about the time in the tumultuous '30s when the Chinese servants of the interviewer's great-grandfather steadfastly waited for their employer for two months at the Leningrad train station, cooking their rice and tea on the platform.

My favorite reader response of all, though, has to be the one from the Texas mother who used my book in her third childbirth. A ''pain detraction,'' she called it, summoning up a role I had never in my wildest dreams imagined for my chef-d'oeuvre. Her husband read the book aloud as she was in labor, she recounted, and it ''worked beautifully'' until a record 20 minutes before delivery.

But, oh yes, dystopia. Well, if ''utopia'' is a vision of a perfect society, ''dystopia'' is a vision of a too perfect society run amok - as in countless sci-fi scenarios - just because it is so intolerant of human diversity.

And no, despite the retroactive pleasures, I'm not going to write another book.