Screaming Penguin - Source Does Matter

I know atrox just posted the article on AOP the other day --and it is a good intro -- but I have been working through AspectJ in Action: Practical Aspect-Oriented Programming
by Ramnivas Laddad the last few weeks.
Too much "don't worry about that, we will explain it later." Otherwise, it is a solid book with many good examples. Even better, some of the examples are code you might actually want to use.
4.5 out of 5.2 penguin heads.
Now that that is out of the way, AOP is friggin cool. The whole idea is basically you can match types of operations such as member assignments, method calls, etc, and creat "Aspects". These are specialized classes where you can say "Do this before you call a method that looks like "calculate*(..)". While AOP isn't as fundamental a shift as OOP was to some, its pretty powerful.
There are some caveats though. I can see code getting very confusing if you have many Aspects layered on a system over time. I still like Dynamic Proxy patterns myself, but this is a more efficient way to do things. It places a lot of emphasis on getting your naming conventions in order. Im also not sure I buy some of the things they suggest -- like transaction management -- as being better with Aspects. However, this is seriously a tool you should have in your bag.