It's been a good year so far for Plone textbooks. New titles have been rolling out steadily. Packt currently lists 10 plus a couple related titles dealing with CMS selection and Python.
I should mention that Packt has just opened the floor for nominations for their 2010 Open Source Awards. Get on over to http://bit.ly/deN0Cn and put a plug in for #Plone. You might just win the doorprize of a new Kindle.
Meanwhile, back on topic, Amazon's list of Plone books is several pages long and as usual I've mined the sales rank data.
The graph is getting pretty busy with all the authors I'm tracking. The last 3 titles (Gimenez & Romero, Gross, and de Alba) are all new since my last posting, so there is no trending information available. Stay tuned for next September's data.
Another set of statistics that I've been tracking are the BuiltWith numbers. Apparently, they sample a couple million websites and attempt to identify underlying technologies. Their 10 Aug. graph looks like this: