Hi Jan,

unfortunately, these commands are in no book. I just looked them up in the ConTeXt source. However, these two books helped me a lot in improving my understanding of TeX and ConTeXt. They are definitely worth their money!

The TeXbook by Donald E. Knuth https://amzn.com/0201134489
Programming in Lua by Roberto Ierusalimschy https://amzn.com/859037985X