I had a nice long Skype conversation with Richard last week; despite an 11-hour timezone difference we managed to connect.
Richard tells me to expect the next volume of 21st Century Checkers at the end of 2013, followed by one volume per year until completion. Of course, these are estimates, not guarantees.
I am also at long last progressing on my reissue of Solid Checkers. I can't give a "due date" but it could possibly be the first part of 2014 (it's slow going and I can't use the computer too many hours a day).