I am trying to find suitable reading material in the form of published books, online articles and guides which cover topics on good design principals for CMS's.
I am looking to add support to an open source PHP framework I am involved with to aid others in the development of modular systems including CMS and additionally the development of an OS CMS based around the framework.
As I am more interested in the theory of developing a modular, extendable system providing components, plugins and modules and therefore do not mind what web based languages the guides relate to as much of it can apply to any other technology. I'm just struggling to find good material.