Don't worry about PHP6 yet, its a long ways off and there will be plenty of up to speed type books when it releases. For where you are at, online tutorials are your best option. Reason being that you are at a level where there are almost endless subjects to cover, far too broad for any book.
As far as OOP goes, I've come to believe it can be learned from a book, but only mastered though use and experience. If you understand how OOP works, just keep using it and things will become more clear as you go on.
PHP in general can be learned from a book or an online tutorial, but is of course only ever going to be mastered through use and experience. Same with C. Same with HTML, same with driving, same with cooking and same with tying your shoes. Books are nice to have for reference though, and php.net is the best site to always have open in your browser, in my opinion.
One of the things that did annoy me when I went looking for a book myself was the amount of books that covered the exact same topics, over and over and over again. This is a variable, this is a conditional statement, these are your equality operators, blah blah blah. Okay, that can all be learned in one online tutorial, done. The books I liked the most as far as learning advanced PHP and related topics are the apress series. In particular, "PHP Objects, Patterns and Practice" for OOP, and "Advanced PHP" for the rest. Great books, really well written and jammed full of real-world style examples and explanations.
I have to plug the Advanced-PHP-Programming too, its great, however as the title denotes for a intermediate PHP'er it goes into some unnecessary things, like advanced version control, reverse proxies and writing PHP extensions etc which isn't wholly related to scripting PHP, although that said it is a great reference book and gives you a detailed insight of how things work under the hood (opcodes etc) which for an intermediate PHP'er mybe confusing.
mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)