I really don't know which books are better, myself I don't have any other than 'PHP pocket reference' from Oreilly, as well as books about Postgresql, Mysql and flash/as which all include sections on php...
I am not a guru in PHP, nor do I know everything about it, but I have a fair amount of programming experience and have been using PHP since mid 2000 just as PHP 4 came out..
However, I strongly believe that it is not all that important to understand everything and know every single function, it is a lot more important to understand the basics of all of it and be able to look up and find the details when needed (example: remembering what parameter position the needle and the haystack has is not important, look it up when you need it).
As for anything involving online stores, ecommerce and such it is most important to understand security issues, including the code and handling of variables, output of posted input, usage in queries, usage in mail, usage in system calls and so forth..
When talking about large sites (high traffic) it is important to have a good feeling of what PHP (and RDBMS's) uses for resources on your system, such as never use posix-regex, use as little perl-regex as possible, write efficient regex's, write efficient SQL queries, don't use double quotes, and so on....
So, I think my point is that any single book about PHP may not take you to another level, as a lot of things depends and indirectly is affected by things around it, such as platform, what libraries are used for this and that and so on..
...and yeah it would be cool if we could by time and never get tired

Then I would have taken some more time to browse the PHP and Zend code to get a better understanding on how some of the basics are done, such as if typecasting when assigning is more efficient and such..