I've been having a look at PHP 5 Power Programming by Andi Gutmans, Stig Bakken and Derick Rethans. In the spirit of open source, there's a free download of the complete PDF too.

The most interesting parts for me was the information on PEAR. I haven't really been following what they have been doing, and it's covered here in mind-boggling detail. However I think because of this deluge of information on PEAR, it shows a bit of bias because it doesn't cover popular libraries outside PEAR such as Smarty and the like. AFAIK, parts of PEAR are not still not PHP 5 compatible too (correct me if i'm wrong).

Given that Andi is a founder of Zend, I was pleased that tone of the book is quite even-handed from a commercial viewpoint. Though Zend products are covered, open source alternatives such as APC, Xdebug and APD are also discussed.

I find that this is a pretty good intermediate book for people who are delving into PHP5 or want to learn more about PEAR.