Whatever option you go for (books, web tutorials, etc), one thing you must do is make sure the resource is up-to-date -- PHP's OO code is very different in PHP4 compared with PHP5, and newer versions of PHP5 have improved things further still. The trouble is that there's a lot of tutorials and books out there which are out of date and give examples using old-style code.
So make sure you find one that is using the latest version of PHP (currently 5.3).
The last thing you want is to find yourself writing code using PHP4's OO style, because you'll be missing out on all the best features, and you may never even know about it.