Hi Dave, Jay,
I’m glad to learn of your unbooks.
I’m writing an algebra book “Classic Math Problems” – about thirty problems and the deep ideas they illustrate. http://www.worknets.org/wiki.cgi?ClassicMathProblems
They will be in the Public Domain. I’m thinking that each lesson will relate to a human value / pattern / deep idea / practical illustration / set of assumptions and I hope to build a large collection. If it works well I want to do more unbooks like that. I like the term!