According to Tim O’Reilly’s always interesting quarterly analysis of the book industry, Ruby is doing extraordinarily well, with a 689% quarterly increase in sales and is now approaching Perl in terms of book sales.
Caveats include the (some would say profound) difference between book sales and use. Most Perl programmers already have accumulated books on the topic. Similarly, C# showed a spike in book sales, but that is almost certainly largely a reflection on the release of VS2005 and not an indicator of a sudden shift towards that language.
Having said that, an important component of programming language popularity is “buzz” and the perception created by a sudden increase in books/seminars/articles. With sites like Digg creating “flash-interest,” and increasing the volatility of the marketplace for attention, “buzz” may play an even bigger role in whatever The Next Big Language is than it did in the success of C++ and Java (two languages whose success was undoubtedly boosted by the amount of associated teaching / discussion).