Probably no novelist today has quite the diversified portfolio of Muriel Spark--she is a writer of assorted talents and often uses comedy to camouflage her more serious intentions. In her new book, which takes place in 1961 and on the marginal terrain of Israel and Jordan, one of the characters observes: ""It's the Arab mentality. They think in symbols. Everything stands for something else. And when they speak in symbols it sounds like lies."" Miss Spark's book is something of a deception too although its prevalent symbol is apparent from the beginning. The Mandelbaum Gate is the midline incision between Israel and Jordan, and also between the old world and the new world. There is just as divisive a streak in Barbara Vaughan; she is half Jewish, half Gentile, and her chosen Roman Catholicism (this faith has been a prominent part of many of Miss Spark's novels) may be forfeited if she marries a divorced man. Temperamentally she is paradoxical-- she appears to be an English spinster schoolteacher now touring the Holy Land; actually she has been capable of considerable sexual passion and an affair with an archaeologist now working in the Qumran area. The story, which pursues a casually serpentine course, involves Barbara's crossover into Jordanian Jerusalem, finds her in and out of a convent, in and out of Arab disguise, and in and out of the company of Abdul Ramdez, a lapsed Arab who, along with his family, conducts an illegal traffic in insurance, spies and other commodities.... The story, per se, has more physical activity than any she has written and it can and may be read as an intrigue with touches of the absurd. Sometimes it's as chimerical as a Dead Sea scroll. But always it has grace, humor and a lively astonishment. And it's such a pleasure to read.