BOOK REVIEW : It’s a Worthwhile Ride Down the Bumpy Road to ‘Portofino’ : PORTOFINO <i> by Frank Schaeffer</i> ; Macmillan; $18; 236 pages

The Beckers, an American family living in Switzerland, take a holiday each summer at a modest beach resort just outside Portofino.

Narrated by Calvin, who is 10 at the start and 13 at the end, Frank Schaeffer’s “Portofino” is a rich brew of cross-cultural comedy. It is enlivened by discoveries and misapprehensions, family squabbles and healings and the dizzy sensibility of a boy whose world opens up in all kinds of poignant and hilarious ways.

If this were all, “Portofino,” would be a reasonably familiar kind of book, even with the charm of Schaeffer’s writing and his genial eye for the detail that has made Italy so ravishing and sometimes so disturbing to foreign visitors. But this is by no means all.

The adult Beckers are not simply Americans in Italy. They are evangelical missionaries employed by a particularly hard-shell Presbyterian split-off to bear witness to European Catholics. Italy, pagan as well as Catholic, is not just a vacation but also an earnest duty.

Dad--Calvin, for all that happens to him over the summers, would not dream of using his father’s first name--barricades himself on the beach behind stacks of religious reading. Every Sunday he holds a service, complete with 45-minute sermon, in their pensione room. Once they get the amiable maid, Lucrezia, to attend, but it is not a success.

Mom, far more outgoing than her introspective husband, does most of the witnessing. Given the slightest opening, she prays with English speakers encountered on the beach and demonstrates her Gospel Walnut Witnessing Kit to strangers on trains. (Out of this prepared walnut she pulls a ribbon divided into black, red and white and symbolizing, respectively, sin, atonement and grace.)

She says a 20-minute grace at the start of lunch, while Calvin counts the peppers in his three slices of salami, and watches the oil from his olives puddle the mortadella and gradually entrap a fly.

“Portofino’s” principal motif, though not its only one, is Calvin’s own struggles to escape entrapment. Far from trying to convert Italy, he is hopelessly converted. He buzzes around, talking to everyone, making friends with beach attendants, Italian families, an old painter and--most specially--an English girl named Jennifer, who is tart-tongued, upper-class and Church of England. He sips wine with other picnickers on the beach and watered whiskey with the painter, sneaks out at night to see Jennifer and slips into the local church to light candles and pray to the Virgin Mary.

In his encounters, explorations and struggles with his family, Calvin is immensely appealing. If Schaeffer’s own intoxication with Italy can lead him to overdo the sensuousness of food, colors and smells, as well as the exuberant warmth of his Italians, his writing is usually sharp and original enough to draw us in utterly.

Above all, he doesn’t simplify. Calvin’s golden adventures have moments of danger. He spears an octopus, is terrified when the wounded creature wraps itself around his head and then experiences the brutality as well as the relief of deliverance. An old beach attendant bites off part of the octopus’ head to free him.

When Jennifer cuts her foot while they are exploring a remote islet, there is a feeling that things may get out of control. Schaeffer, by turns, is sentimental, celebratory, evocative and very funny, but we are never far from a sense that harshness and violence are real; we are never entirely sure how things will turn out.

This is true, most of all, in the portrait of Calvin’s family. His parents are funny and eccentric, but they are far more than a crotchety backdrop to the boy’s story. They are complex and unexpected; our image of them shifts gradually as we go along, not to a conclusion, but to an absorbing inconclusiveness.

Dad is petulant and kind by turns. He is Old Testament severe, and when he learns that Calvin has been tippling, he beats him brutally with a belt. Yet he breaks out of his aloofness to take his son on an occasional companionable hike. In a sense, he allows him more freedom and gives him more genuine understanding than Mom, who is gentle, infinitely understanding and unstoppably intrusive.

Her fey saintliness goads her husband’s irascibility; he makes terrible scenes. At one points he insists they all pack up and leave at 4 in the morning; there is a wonderful picture of the family waiting at the bus stop until sunrise, when Dad suddenly relents and apologizes. He is not entirely what he seems; he is both scarier and more likable. And so is Mom, as it turns out.

Schaeffer makes this utterly unpredictable family--besides the parents, there are Calvin’s two sharply drawn older sisters--both painful and appealing. The Beckers are immensely present and finally ungraspable. Their vivid pieces don’t quite fit together, and neither does life. “Portofino” is bumpy and can swerve briefly into winsomeness or staginess before bumping back. But bumpy roads offer the best and most unexpected views; certainly this one does.