|I have to confess that I do not use Eclipse. It is an excellent tool but it consumes a lot of memory. The problem it is that it written in Java and need a lot libraries to run. The same goes for Zend Studio.|
Since I am running Linux, I use Kate. It is much simpler for my needs.
As for the book utility versus online documentation, I am not sure, but I found the book very handy when I tried using PHPEclipse.
Besides being a printed book that you can carry anywhere, I found it useful in particular the chapters about setting up to work with PHP debugger extensions, which is usually an hell with most other IDEs that I tried.
I also liked the chapters that cover other interesting extensions for accessing repositories in CVS/Subversion and relational databases. Those extensions are unrelated to PHPEclipse but they are really helpful for everyday PHP development.