ZEND PHP 5 Certification Study Guide
The book is around 250 pages long (short by most standards). To this point I have found that this book is a good way to brush up on things I already knew and learn some things that I did not. I am personally using the book to help me study for the PHP 5 certification. However, certification or not, it is turning out to be an excellent tool for honing my skills. It has the added benefit of showing me areas that I need to work on as well as indicating to myself how much about PHP I really know.
I would recommend this book to the PHP programmer of any level.