THE SECOND WORLD WAR is winding down. On

an unidentified island in the Pacific, in a hospital to which wounded Australian soldiers have been evacuated is a special ward--Ward X--reserved exclusively for those soldiers whose wounds are not physical but psychological. They are "troppo," the Australian word for soldiers who have broken under the stresses of warfare. Not "crazy," in the usual sense of the word, but not normal either, they are, to put it kindly, mentally deranged, some more so than others.

Sister (Australian for nurse) Honour Langtry, a 30-year- old unmarried woman from an upper-middle-class Australian family, is in charge of Ward X. Since the ward is physically separated from the rest of the hospital and since there is no staff psychiatrist, Honour takes, by default, full responsibility for the five patients who remain there as the war ends and the book begins. They are Matt, who has, supposedly, "hysterical" blindness; Nugget, all but immobilized by severe hypochondria; Benedict, withdrawn and depressed as a result of having helped wipe out a village whose only occupants were women and children; Luce, a bisexual actor from the slums who has been consigned to Ward X because he was blackmailing his commanding officer and the army could find no other convenient way to dispose of him; and Neil, the only officer of the five, who is guilt ridden because in some way (never clearly defined), after five years as an extraordinarily capable commander in jungle warfare, he finally made an error that resulted in the loss of some of his men. No one blames Neil but himself; and that is quite enough.

The five men and Nurse Langtry have established a precarious balance among themselves. Their lives are tranquil, each day much like every other day. Nurse Langtry and Neil are attracted to one another and there is an unspoken agreement that, once the war is over, they will probably marry. By mutual consent--so as not to upset the relationships between Nurse Langtry and the other patients--there is no overt romance. All five men, only too aware of their own and of each others' deficiencies, lean on Honour Langtry. She is not only their Nurse but their lover and mother figure as well.

It quickly becomes apparent that Honour Langtry realizes the power she holds is potentially dangerous. She has come--though she never admits it, even to herself (I am not even certain that McCullough realizes it)--to love her position. She might have been a doctor. She had the necessary credentials, and her family could have afforded to pay for her education. But she preferred nursing because she liked to spend her time with patients. Now, on Ward X, Honour Langtry has the best of both possible worlds; the power to make decisions as a doctor would and the obligation to provide the intimate, day to day personal care that is a nurse's responsibility.

To this relatively stable mini-world, another patient, Sergeant Michael Wilson, is introduced. As the reader might guess, Wilson upsets the precarious balance. He is a truly "good" man, self-sacrificing and almost saint-like, a man who feels responsible for all his "brothers." He is also a man with a temper. He once "broke," and nearly killed his commanding officer. Now he wants only to be a peacemaker. Unfortunately for the stability of Ward X, his looks and intelligence make him irresistible to Honour Langtry, though he makes no attempt to seduce her. Luce, an obnoxious, self-serving, unscrupulous rotter, who has been behaving in what, for him, is a relatively respectable fashion, now shows his true colors. Emotional upheavals of all sorts occur, ending in the violent death of one of the patients and near disaster for Honour and all her charges.

An Indecent Obsession is never boring. Colleen McCullogh has created fully drawn and believable characters, and she keeps her plot moving along nicely-- though I do wish she could have introduced a bit of levity. There isn't a laugh in it. But the author also delivers a message, intended or not. It is that happiness, contentment and tranquility depend to a great degree on how we manage our relationships with our families, friends, lovers--all those who are close to us. For example, if we get too close to one parent, the other may be hurt. If we spend too much money or time on one child, another may be jealous. If we see too much of our friend, another may feel deserted. We're not always as sensitive as we should be to the effect that any one of our relationships may have on another; our worlds are too large. Fortunately, most of those with whom we come in contact have the basic emotional stability to understand or overlook moderate shifts in our emotions. But in the microcosm of Ward X, with all its inhabitants already on the thin edge, the delicate balance of interwoven lives, and their frightening fragility, becomes terrifyingly apparent. In fact, if Edward Albee hadn't already appropriated the title, A Delicate Balance would have been a more illuminating title for McCullough's book than is An Indecent Obession.

Finally, the question a lot of potential readers will want answered: Is An Indecent Obsession as good as McCollough's The Thorn Birds? The question can't be answered. It's like asking if a nice, ripe orange is as tasty as a nice, ripe apple; it depends on your mood and your taste buds. I enjoyed both books, but I thought An Indecent Obsession was more intriguing, more thought provoking, than was The Thorn Birds. But then, I also prefer scotch to bourbon, swordfish to lobster, and Robert Benchley to James Thurber. "Cum gustibus non disputandum est"--"there's no arguing with taste"--as they used to say in Rome. #90:By William A. Nolan; WILLIAM A. NOLEN, author of A Surgeon's Book of Hope, practices surgery in Litchfield, Minnesota.