>Take a look at "UML Distilled - Second Edition", written by Martin Fowler. Really nice.

I'll second that - it is a very useful book. It does have it's weaknesses. I think his example in the final chapter is unnecessarily complicated. I have found that to really get what he's talking about, I have to learn the whole project from scratch each time. Still, I'll agree that it is a good book.

Also check out The Unified Modeling Language User Guide (Booch, Rumbaugh, Jacobason). It's a good supplement to UML Distilled. It goes into much greater detail. It talks less about technique, and more about the UML. It too has it's weaknesses - those guys just aren't the clearest writers - but overall I have found it quite helpful.