Maybe I missed it in the article, but what online tool were they using to develop the textbook?
The tool is called CK-12 FlexBooks, and the link
is near the bottom of the submitted article.
I'm actually not a big fan of the text. I skimmed it a bit and I don't think it's as well written as one might hope for. In particular, I hate when formulas just appear with no explanation of how they where derived, and in sections 1.2 to 1.3, the use of big names such as "Fundamental Counting Principle" makes stuff sound more intimidating than it is. I think the discussion about the cardinality of the sample space when you have events that are order sensitive vs order insensitive (ie just counting permutations) could be clearer. Nonetheless, it's an awesome start and the teachers involved should be congratulated.
[1] http://ck-12.org
[2] http://www.ck12.org/about/tutorials
[3] http://print.ck12.org/www/2630552d0bf13d39a2850c0e9f0a523f.p...