Over the past couple of months, two new books have come out on Plone:
Plone Content Management Essentials by Julie Meloni available from SAMS.
The review on Slashdot is very thorough and interesting.
Plone is becoming a real market leader in content management software. Consider that:
"The UK's Ministry of Defence's Defence Academy says it chose Plone and Zope, an open source content management system and supporting application server, for the software's functionality, not its negligible cost." (Infoconomy Article)
Two major newspaper sites (Boston Globe and the San Diego Union-Tribute) use Plone
The chief objection that I hear about Plone is Python and lack of in house skills. I have a feeling that with books like this, this trend may change. I wonder if Python and Zope could be the de-facto technologies for content management. Given that Zope has been designed from the ground up as a content management focused application server and Python is an extremely effective and high performing programming language, I would not count the possibility out.