"The Power that Preserves" is amazing. Astonishing. Breathtaking. The sort of fantasy novel that you expect to find once a decade, if it's a particularly good decade. This is the culmination of a story that so big you can barely believe it fits in three books, so intense that it seems amazing that any person could actually have written it. It is a story you will never forget.
One portion of the novels follows Lord Morham as he attempts to save the Land from total destruction in a final battle against Lord Foul's forces. The giant reaver Satansfist has Morham and the other wizards and defenders of the Land under siege at the castle of Revelstone. This contest is in amazing piece of virtuoso writing, perfectly melding several great action sequences with intense psychological passages to create something dazzling. I feel perfectly comfortable saying the Donaldson is the only fantasy author who has ever equaled Tolkien in writing battle scenes. The emotional triumph at the conclusion of this one is, well, words just can't describe it.
Any lesser author would have devoted an entire book just to that. For Donaldson, however, it's just the prelude to the main event. The main event, it's no spoiler to say, is the final showdown between Thomas Covenant and Lord Foul. And what a showdown it is. When Donaldson started writing this trilogy, the most important decision he made was that his main character would not be a standard fantasy hero. Thomas Covenant, of course, is the most deeply sympathetic and real character ever to appear in any fantasy novel. Having created a person as amazing as this one, of course, poses the problem of how to provide a satisfying conclusion while still remaining true to the character. Probably nobody alive could have pulled it off as well as Donaldson, but then again no one other than Donaldson would ever have attempted a fantasy series this ambitious anyway.
So in conclusion, read it, love it, be amazed by it.