As pointed out on the Professional PHP Blog today, there's an interesting set of graphs on the book sales for different topics, including Ruby, Java, and yes, PHP.
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.
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 isn’t PHP the natural successor to Java. A small cadre of agile minded developers are giving Ruby a spin.
And, of course, it's nice to see - as he also mentions - that PHP books are still outselling Ruby books by 3x. Of course, that's a double-edged sword, with Java books outselling PHP books by 2.5x.