THE THORN BIRDS
by COLLEEN McCULLOUGH 530 pages. Harper & Row. $9.95.
Well before its publication, this second novel by a hitherto obscure Australian had become an economic event. Harper & Row ran off 225,000 hardback copies and put up $100,000 as an initial advertising budget. The Literary Guild made the book its main selection for June, relegating Erich Segal's Oliver's Story, a dead-certain moneymaker, to second place. Avon Books shelled out $1.9 million for paperback reprint rights, topping the record $1.85 million that Bantam Books paid for E.L. Doctorow's Ragtime two years ago....