Recommended Books on Object Oriented Programming

Hi, I want to properly learn object oriented programming as I've been coding in procedural style since I started with PHP a few years ago, and want to give OOP a shot. The web isn't really a good resource to learn OOP in PHP to be honest, as a lot is outdated for PHP4's style of OOP. I've looked into OOP quite a bit and understand the concept of it, and want to take it further. Any recommendations appreciated :). Thanks, Ben.