BOOK REVIEW / FICTION : A Love That Takes Him to the Ends of the Earth : THE RIDERS <i> by Tim Winton</i> , Scribner $20.95, 384 pages

Halfway through this metaphysical thriller by Tim Winton, a 7-year-old girl named Billie wonders “if you could love someone too much. If you could it wasn’t fair. People didn’t have a chance. Love was all you had in the end--when you fell off the world there was still love because love made the world.”

Billie’s father, Fred Scully, has fallen off the world. He has worked feverishly to fix up the dilapidated 18th-Century peasant cottage in Ireland that his wife, Jennifer, decided to buy on a romantic whim. Meanwhile, she was supposed to be selling their house in their native Australia. Having completed the work in bitter December weather, Scully drives to the airport to greet his family. Billie gets off the plane alone. No Jennifer.

Moreover, Billie is in shock and can’t or won’t tell Scully what has happened. He finds out that Jennifer accompanied the girl as far as London, put her on the plane for Ireland and disappeared.

Where would she have gone? Greece, he decides. That was where Jennifer had seemed happiest during their years as wandering expatriates in Europe--years when she tried to write and paint and shed her conventional upbringing while he cheerfully supported them with any labor that came to hand: fishing, painting houses, plumbing, lugging stones for a mason.

Scully is a likable bloke, despite his wild hair, “wonky eye” and face scarred in a brawl on a fishing boat. Billie compares him to the Hunchback of Notre Dame: “Not very pretty. Sometimes he wasn’t very smart. But his heart was good.” Surely he doesn’t deserve to be abandoned so suddenly and cruelly--if this is indeed the case.

He wants answers.

So the chase is on, from Greece to Italy to Paris to Amsterdam, in the last days before Christmas. The old acquaintances Scully queries about Jennifer are either hostile or evasive. His optimism and acceptance of life, it seems, have bred a covert resentment in others. But there is always another clue, another hope dancing just out of reach, as Scully flogs his only credit card to board yet another plane, boat or train, dragging Billie with him.

Winton (author of “Cloudstreet” and the “Lockie Leonard” books) keeps the suspense taut until the very end. He is a wonderful, descriptive writer who seems incapable of a slack or routine paragraph. He makes Ireland so vivid that we can’t wait for Scully to resume his life there after what we--and Scully himself--expect will be a momentary interruption; later, in equally vivid Continental locales, we half forget that Winton is after bigger game than suspense.

Scully has seen ghosts. They haunt the ruined castle near his cottage. They are medieval riders with torches, cold, exhausted and bloodied, who gather before the dark and silent keep, awaiting some message that never comes.

And the chase, we finally realize, despite all its realistic detail, isn’t unfolding in a realistic way. Scully is part Job, part Candide. He is the Hunchback collapsing in Notre Dame on Christmas Eve for love of his Gypsy girl. He is Joe Btfsplk, the Al Capp cartoon character with the cloud over his head, only his rain falls on other people too.

Billie is mauled by a dog. A has-been painter who once taught Jennifer commits suicide after Scully visits him. Then a woman named Irma--a drunk, S&M; addict, maybe a thief--claims spiritual kinship with Scully and Billie and chases them; Scully, in the depths of his desperation, betrays her.

As for beautiful, black-haired, perennially dissatisfied Jennifer (whose identity, at times, Irma seems to appropriate), she is la belle dame sans merci . If only he had known.

Scully wants answers, and so do we. But Winton poses a different question: What if there are none? Can Scully get on with his life and take care of Billie, or must he become one of those riders eternally waiting before the castle, “whose light did not show--patient, dogged, faithful in all weathers and all worlds, waiting for something promised, something that was plainly their due”?

We don’t literally believe in the ghosts, just as we don’t always believe in Billie’s precociousness. But Scully is real. His hope and his pain are as real as a slant of light, a hangover, a laugh, the smell of dirty socks; and we follow this pilgrim’s progress with a heart-catching sense of our own soul’s fragility.