Most of these are outlined with Andre Norton, written by the co-author, with Andre Norton reading through the final draft.

One exception to this style, are the current SOLAR QUEEN books with TOR. (These are done with very little input from Andre Norton.)

The other execptions are TIGER BURNING BRIGHT where each character is written by a different one of the co-authors and, BLACK TRILLIUM where each character was written by a co-author, and then the editor put the whole thing together.

I think that you will find that each of the separate co-authors put their own stamp on a book. In most cases these are books that Andre Norton would never have written, never had the time to write, and never would have returned to that world. The co-authors allow several 'old' worlds to be explored.