Have been thinking about doing one more book, though might not be able to stop there. I also like Horse and Buggy West a lot, but some (including Eileen) have suggest something more like O'Connor's The Last Book.
John,
I have read somewhere the H&BW caused a lot of hard feelings among the descendants of some of the Arizona Territory families that JOC wrote about, so you might want to think twice about writing your version of H&BW.