Most of the Norton books I own belong to this series. I have a complete collection of titles in the series (though not all editions; a long way to go there).
*A word on editions: my definition of edition would be a book with a different binding, size, or cover art. I also consider a book a different edition if it uses a significantly different typestyle for the title/author; for example, see the 1968, 1970, 1975 and 1978 US editions of Sorceress of the Witch World. All have the same cover art, but use significantly different logos/typestyles for the author name and title, or are different sizes (the 1968 edition is smaller).
The collection is organized by cycles and series (Estcarp Cycle, High Hallack Cycle, The Turning, Secrets of Witch World) followed by collections of Witch World short stories, and other books which contain at least one Witch World short story or novella.
*Witch World was originally called Quatpe of Swords. That is according to Len Collins in an article entitled "The World of Andre Norton" in a 1964 Canadian fanzine called TTH. Thanks to Paul Goode over at the Andre Norton forum for this tidbit.
*Koris of Gorm was modeled on a historical person. An interview with Andre Norton by Willard Harrison was printed in 1974 in Huitloxopetl #9, a fanzine published by Meade and Penny Frierson. Harrison asked Norton if any of her characters were based on any person, living or historical, and Norton replied, "No. I have an idea in mind, but once I begin writing, the main characters take over on their own, often changing in the process. I once had a villain turn into a second hero. There is only one character I ever directly based on a historical one and that is Koris of The Witch World who is modeled on Bertrand du Guesclin, the abnormally ugly hero of French history." That is also from Paul Goode, who notes that du Guesclin lived from 1320 to 1380.
**Note: pb = paperback; hc = hardcover