O’Reilly radar has an interesting graph of 2004 versus 2005 book sales for a variety of languages. The big news is that Ruby books sales are up 1552% and Java book sales are down 4%. This would be consistent with my observations from Why isnâ€™t PHP the natural successor to Java. A small cadre of agile minded developers are giving Ruby a spin.
Meanwhile, there is no change in PHP book sales. Perhaps this suggests that Java is loosing to Ruby, but PHP is not? Or PHP is gaining from Perl, the other language with declining sales, just as fast as its loosing to Ruby? Perhaps you can’t read anything much at all into these numbers.
Also mentioned is that Java books outsell PHP books by 2.5x and PHP books outsell Ruby books by 3x.