What I found most interesting there is that, if the 100% annual growth continues, Ruby will overtake Java in a little over a year, and become the most popular language (based on books sold) shortly after that.
Of course, it looks like ruby is slowing down, so that might not ever happen.
Also, since Groovy came from nowhere, it's growth rate is basically infinite. If that trend continues, by next month every atom in the universe will have bought a Groovy book...
Interesting data. My first reaction was that lisp and scheme are still underutilized and underappreciated.
But then, the metrics he's using wouldn't count downloads of the PLT books, or Shriram Krishnamurthi's Programming Languages: Application and Interpretation, and it's not clear SICP or EOPL would be counted as Scheme books in their reckoning.
And that's assuming you accept popularity as a useful metric.
Of course, it looks like ruby is slowing down, so that might not ever happen.