I emailed Tan books and asked what kind of a demand they would need because it seems there is a high demand for these books. They are impossible to find used. This was her response below. So, if anyone is interested, let me know. There is no commitment to buy; I just want to show them the demand.
"We would probably need to see a substantial increase of interest in the books 11-20 for an extended period before our production department would consider going to print on the 21+ books. Hopefully in the future we’ll be able to carry the full line, but I’ve been told it isn’t a practical reality for our production schedule currently. Thanks for your interest though! It was funny, yesterday afternoon I had someone call about the books 21+ also, after I had replied to your email. Perhaps if I received several more calls over the next few months I would be able to put some pressure on our production department." ;-)