UNDER FIRE: AN AMERICAN STORY. By Oliver L. North, with William Novak, HarperCollins/Zondervan, 446 pp., $25READING "Under Fire" is a bit like having Oliver North sit down in your living room (yes, you first have to open the door and let him in) and say, "About Iran-contra, let me try to explain...." The explanation is energetic, often humorous, sometimes touching, and occasionally bitter - but never boring. The book begins on a bizarre note. The acknowledgments describe how the publisher organized the process of writing as a covert operation, with William Novak and Mr. North meeting surreptitiously every Monday in a hotel room to work on the book; when room service brought meals, North would hide in the bathroom so he wouldn't be seen. After that odd but telling distraction, North begins his story with his dismissal from the National Security Council and his surprise at being described as the only government official with direct knowledge about how Uncle Sam was helping to put the contras' hand into the Ayatollah's cookie jar while also trying to secure the release of US hostages held in Beirut. He then details the circumstances under which he became involved in the arms-for-hostages swap, initially to handle the logistics of the operation. He says he had doubts about the idea; it ran counter to the administration's antiterrorist policy, which he helped formulate. "For me, the most difficult aspect of the endeavor was accepting that we had established a price for a human life; 500 TOW missiles. To this day, I find this part of our Iran initiative to be the most troubling." With the release of hostages Terry Waite and Thomas Sutherland this week, allegations have surfaced in Britain that the CIA used Mr. Waite to try to free its Beirut station chief, William Buckley. A columnist for the Times of London reportedly wrote that North met five times with Waite. North has denied allegations that Waite knew about the arms-for-hostages swap and that he gave Waite a signaling device to help locate the Briton if he was captured. "Under Fire" sheds no light on these allegations, alth ough North mentions Waite in passing, noting his courage in trying to negotiate the release of hostages. Because of their intimacy, autobiographies often elicit sympathetic responses from readers. This one is no exception. Much of that sympathy can come from the story itself. North's story clearly is one of rising from modest beginnings and through a good deal of adversity to an influential White House post. But much also comes from presentation: It begins with North as victim, moves to North as humanitarian, establishes his credentials as a caring father who had to rebuild his marriage almost from scratch, records his Christian rebirth - and only then does the story move to the more contentious issue of the Reagan administration's support for the contras and its attempts to circumvent the congressional ban on aid to the rebels. "Under Fire" is worthwhile reading: It opens yet another view on events that are still playing themselves out.