I would recommend ML for the Working Programmer (available free online on the author's site). Learning ML forced me to write in a properly functional style rather than writing "C in $LANGUAGE" (the book doesn't even mention the possibility of using mutable variables until quite late on); by the same token I'm not sure the language is that usable in the "real world" (I work in Scala these days), but the lessons from it were useful even when I was working in Python.