BOOK REVIEW : Travelers Done In by Fellow Americans : HILLTOWNS; <i> by Anne Rivers Siddons</i> ; HarperCollins; $22, 468 pages

Though the adventures of wide-eyed Americans in collision with the sophisticated Old World remains an alluring theme for a novelist, the supply of suitable innocents is rapidly dwindling.

When Mark Twain and Henry James explored the subject, the grand tour was the exclusive diversion of a privileged few, but a century of major and minor wars, cheap air fares and the late, lamented sexual revolution have all combined to turn the totally naive American into an endangered species.

These days, finding a set of middle-aged, middle-class Americans who have never left home in any sense of the word can be a real challenge. The contemporary writer has a lot of back story to tell before getting on with the plot.

In “Hilltowns,” Anne Rivers Siddons has given Cat Compton Gaillard and her professor husband, Joe, a melodramatic pair of reasons for remaining permanently in the Tennessee college town where Joe teaches literature. Cat is not only an agoraphobe, but the Gaillards’ only child was born blind. When Cat was 5, her parents were killed in a preposterously lurid auto accident.

Since then, she has been morbidly afraid of the unknown, restricted to the tiny town where she grew up.

Now, however, the Gaillards’ self-sufficient daughter Lacey is having a glorious time touring Europe with friends, effectively removing Cat’s best excuse for ignoring her phobia. For 20 years, Joe has been devotion itself, never leaving his wife’s side for more than a night.

A skeptic might wonder why he has accepted the status quo with such cheerful stoicism, but pop psych has made the symbiotic relationship a household word. Joe needs a helpless dependent and Cat needs a protector.

When their two closest friends invite the Gaillards to Italy for what promises to be a fantastic wedding followed by a honeymoon trip through the hill towns of Tuscany, Cat decides it’s time for a cure.

After intensive psychotherapy, she declares herself ready to go. They’ll see Maria and Colin married in Rome and then the four of them will take off for the countryside.

Cat reminds herself that the city phase of the trip will be short and they’ll soon be in hilltop villages as small and safe as her Tennessee enclave.

Their arrival at Rome’s airport from hell is an inauspicious beginning. The electricians are on strike, and Joe’s luggage is lost with all his clothes and Cat’s chic new outfits, a trial she weathers better than he.

That night, jet lagged and testy, they go to a glamorous pre-wedding party at the home of their hosts, the celebrated expatriate painter Sam Forrest and his dazzling, ultra-worldly wife, Ada.

Feeling gauche in her flowery sun dress and ignored by Joe, Cat drinks too much and finds herself in intimate conversation with the great Sam Forrest himself, a huge, coarse, but surprisingly sympathetic fellow Southerner.

She awakens with a monstrous hangover and surprisingly pleasant memories of Sam, so utterly different from her conventional husband.

Almost immediately thereafter, the bridegroom, Colin, breaks his ankle in a foot race with Joe in the Circus Maximus. Joe gets off relatively lightly with heatstroke. The Forrests have an instant solution for the blighted trip.

Because Colin will be unable to drive, they offer to join the tour with their car, and since there will now be plenty of room, they include an old friend, the TV personality Yolanda, an earthy redhead with a serious promiscuity problem.

At this point, the novel departs from the traditional 19th-Century pattern. Cat and Joe will not be corrupted by decadent Europeans, but by their fellow countrymen altered by extended sojourns abroad; a second-hand impact, but no less powerful or life-altering for that.

The hill-town itinerary is curtailed by further medical catastrophes, and the Gaillards find themselves spending most of their time in Rome, Florence and Venice. In this steamy and hectic atmosphere, tensions escalate and tempers fray.

The seven-person honeymoon functions as the perfect incubator for everyone’s repressed neuroses: sexual, psychological and professional.

In need of fresh inspiration and enchanted by Cat’s ethereal beauty, Sam is painting her portrait, a project that requires them to spend hours alone while Ada entertains Joe.

The newlywed Maria and Colin, turned into patient and nurse by Colin’s accident, have a premature introduction to the literal meaning of “in sickness and in health.”

Geographically, “Hilltowns” doesn’t ascend much beyond Siena, nor does it introduce us to more than a few token Europeans, but never mind. The personal interactions among the seven Americans provide more than enough material for a thoroughly modern version of “Innocents Abroad” or “The Ambassadors.”