Check out "PHP 5 CMS Framework Development" by
Martin Brampton. Came out last year, and I've been using it to help whip
my own custom CMS framework (for real estate web sites) into shape. If I
remember correctly he used to work on Mambo (which then spun off into Joomla)
and has since been working on his own CMS, named Aliro. This book is
basically looking back at the way he set things up for Aliro, and while I
haven't used all of his suggestions it was very enlightening, and a lot of it
applied to my own system.