Mastering R; see here:
R: What is R?
Going back to mastering Perl. Perl’s creator, Larry Wall, once described his language as “a cleaned up and summarized version of that wonderful semi-natural language known as ‘Unix.’”
Don't read much about TcL and/or Expect these days, perhaps I'll resurrect conversations about these awesome languages.
Currently re-reading the book, Minimal Perl For UNIX And Linux People (2005), just for fun.
I recall reading a section of the original K&P's The UNIX Programming Environment about social engineering, think I'll re-read that in parallel.