On PHPClasses.org there's a recent book review of a new offering from Wrox Publishing - "Expert PHP and MySQL" (by Andrew Curioso, Ronald Bradford, Patrick Galbraith). Udi Mosayev is the reviewer.

This book is for developers who have worked on several projects and have some good experience developing with PHP and MySQL, as well developers who develop high-demand applications or applications that handle lots of data and processing. OOP experience is recommended.

He talks about the subjects he specifically likes that the book covers like opcode caching, general caching techniques, Gearman and memcached.

The techniques discussed in this book are used by large sites that we all know like Digg, Yahoo, Facebook, YouTube and more. If its good for them, I believe it is good for you too.