There's an interesting new post over on the O'Reilly OnLamp.com website containing some statistics about their book sales for the past few years by language.
Recently there was a thread on the O'Reilly Radar started by Tim O'Reilly posting a chart that I put together from our book sales data. The chart showed comparative market share for most of the relevant programming languages. I have updated this in the chart below and have included 2005 data as well.
The chart shows the trends each of the languages' sales has followed over the course of the past years. PHP's on the last and seems to have held pretty steady in a time when very few seem to be trending upward.