Stepen R. Donaldson is one of those rare writers who is able to write both excellent fantasy as well as science fiction.
Most people consider Thomas Covenant, the unbeliever, a very unpleasant person. But I think that Stephen did an excellent job. Thomas, but also the other characters are very alive and human. The first chronicles consist of the following three books:
Excellent fantasy story and highly recommended. A part that was left out of the "The Illearth war" has been made available as a separate story: Gilden-Fire.
If you like the first chronicles you might like the second one even more. The second chronicles consist of the following three books:
There are four books projected, the first one, "The Runes of the Earth", is planned for fall 2004. The first chapter of the prologue is available on the official Stephen Donaldson website.
Excellent science fiction story, the kind I like most. Even more lifelike compared to the Thomas Covenant books. However, if you think Thomas is not a nice person, beware since there is a lot of violence in this series, especially the first book: The Real Story.
To me it's a part of the story, and moreover, part of the characters. They are extremely convincing.
I recently reread the Gap series, after 10 years, and was again amazed by its very high quality of writing by Stephen R. Donaldson.
The first time I read the first book, The Mirror of her Dreams, I got stuck very fast. It felt like Donaldson had reused the story line of Thomas Covenant somehow. In 2004 I bought both books (second hand) and decided to give them another try and I was rewarded.
If you consider the Chronicles a bit too harsh, try Mordant's Need. In some places it reminded me a lot of Zelazny's Amber books, how mirrors are used and maybe even the overal story. I really liked both books a lot, recommended.