Ant is a java build tool that has pretty much become the standard way to build java applications. I've dabbled in it but figured a bit of immersion therapy would be useful.
The book covers version 1.6.1 and 1.7.0 is the current version. Fortunately, the amount of change is relatively minor. It's a decent book that goes into ant starting from a basic level and progressing to a moderate level of detail, I wouldn't call it "definitive". It is worthwhile, but it hardly goes into the kind of advanced usage that the word "definitive" implies to me.
However, if you aren't an ant expert, it is a worthwhile read. If you are already very comfortable with ant, it probably has little to add.