[eluser]n0xie[/eluser]
The userguide is one of the best online documentations I've ever come across. I don't think a book would give you much more information than you'd get from the userguide. Then again I'm used to scrolling through manpages so I don't know if that's a fair comparison.
Anyway, the best you can do is read a good book about PHP5 OOP and then all you'll need is the guide to lookup certain methods or libraries. I use it more as a reference cheatsheet then as anything else...