I'm in the "UML as sketch" camp, and highly recommend "UML for Java Programmers" by Robert C. Martin as a good introduction. "UML Distilled" by Fowler is a good classic which goes into more detail.
The best way to learn UML is to use it and to discuss your diagrams with others. Coming to JavaRanch was a good first step...
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
An important thing to remember is that the UML isn't sufficient for modeling business applications, so that you need to learn more than just UML modeling techniques. At Agile Models Distilled I overview a wide range of techniques.