IT MAY NOT BE true, but I have the feeling that Muriel Spark is one of the few, in the category of fine writers, who has a grand time at her work. This is not because she writes comedy. It would be wrong to believe that people who write comedy sit down to their desks already laughing. However, readers who have enjoyed her novels from the early Robinson and Memento Mori may suspect that Muriel Spark sits down to her desk already malevolent. In her new book, Territorial Rights, there is hardly a character one doesn't enjoy a little, but the whole lot of them are rotters, really, really rotters. And in the end there is the suggestion of a distribution of their fates which is neither just, nor actually unjust, but is funny.

The story opens in the Pensione Sofia in Venice. It is fall, past the season, the canals choppy. Robert, a young Englishman, seems to have fled Paris to find refuge here, and to track down Lina, a poor Bulgarian girl (of 35) who has the habit of stuffing her household rubbish under her skirts and shaking it into the canal. This engaging girl had defected from her country and after a spell in Paris, living on a refugee grant provided for true dissidents, has come down to Venice to find the grave of her father, murdered in 1945: the center of the mystery.

Pursuing Robert is the aging American millionaire, Curran, who had kept Robert as his lover for a couple of years, and now settles in at the Hotel Lord Byron to see what he is up to. Curran is the sort of international aristocrat/confidence man who knows everything, knows everybody. And by coincidence, a device Miss Spark uses joyously, there also arrive at the same pensione, Robert's father with Mary Tilly, a middle-aged woman in a fur coat and tight boots whom Robert immediately spots as not his mother.

The father blusters nervously, ""A little holiday," he said. "My former colleague and I are having a little holiday. Here in Venice."

""Oh, your colleague..." said Robert, with the cruellest of courtesy. The elderly man suddenly gave his son a look of disgust."

All of the characters who gather have reason for mistrust, and indeed there is a skeleton (cut in two, the parts buried separately) in the past. Robert's mother, getting wind of the gallivanting in Venice, resentful of being left behind, is so mistrustful that she enlists the services of an investigative bureau, its agent for northern Italy being the Countess Violet de Winter.

"Over the past ten years her business...had deteriorated by seventy-five percent largely because unmarried lovers no longer chose Venice as the most desirable place to be together and, moreover, the lovers' husbands and wives no longer seemed to care if they did. "The bottom has fallen out of the love-bird business," she frequently told her old friend Curran...."

In Territorial Rights it is the Bulgarian Lina who has some of the old Spark spark. Lina receives a letter withdrawing her refugee grant: "The group feels this way. Don't be upset, Lina, but you know you never answer letters, sign petitions or come to our meetings and our demonstrations. Additional to this, there has been a report that you are not seriously a Dissident. Isn't it true, Lina, that you believe in nothing and know nothing of our struggle? Please do not take this personally, but you should never have left Bulgaria. Nobody was persecuting you, You do not suffer. You do not share our aims...."

Lina is indignant: "They say I don't suffer and fight. Those dissidents who stay at home are the ones who suffer. What have we here got to suffer about?""

On the other hand, when she finds herself in the middle of American tourists admiring the mosaics in St. Marks, she announces loudly, ""We have also in Bulgaria."

It is not only Lina who believes in nothing. None of the characters has even a rudimentary sense of conviction, of moral integrity, of compassion, and this is not the case in most of the stronger novels Spark has written. Memento Mori was so funny, so wicked, it could do without. In a recent interview Spark spoke of her own writing as "pretty harsh and detached. Cold, even. I like to save the warm bits for when I mean it. I'm very careful with the prose."

Well, there are no warm bits in Territorial Rights, and while it is not difficult to read - Spark has a bold wild way with a plot - the flesh, the wit, the bizarre vision of truth one anticipates in her fiction seem temporarily exhausted. In the interview she was asked about the ups and downs in a writer's life and she answered, "The best thing is write on, regardless." It may be the best thing for the writer. It is not, in this instance, the best thing for her admirers.