I was checking on the side books today instead of the main run. I always forget the Friends of the Horseclans books are not just regular numbered novels.
The list I found says there were two anthologies, Friends of the Horseclans in 1987 and Friends of the Horseclans II in 1989, with stories in the Horseclans setting by Robert Adams and other writers. ISFDB also lists them as anthologies and credits Pamela Crippen Adams with him.
I have not spent as much time with those as the main books, so maybe I should dig them back out. Do folks count the anthology stories as must-read Horseclans stuff, or more like extras if you already like the world?