UML 2 Toolkit - For those of you with some UML skills but looking to catch up on the UML 2 updates, I recommend the book UML 2 Toolkit published by OMG press. Make sure you get UML2 Toolkit and not UML Toolkit. One of the reasons I like the UML2 Toolkit is that it does a good job of making clear the changes/additions/etc. in the UML2 - most chapters have a section at the end titled "Relevent Changes in UML 2".
n.b. I know two of the author and am mentioned in the credits of UML 2 Toolkit but do not profit from the sale of this book. UML User Guide, 2nd Edition - Updates the widely popular text on the UML. Be sure you get the 2nd edition for the UML2 material. Covers almost all of the UML - even the parts you will rarely use. UML Distilled, 3rd Edition - As the title says, a distilled version of the UML and a great one as well. Focuses on those parts of the UML you will use most often. Be sure you get the 3rd edtion for the UML2 material. n.b. Booch et al. in the UML User Guide and Fowler in UML Distilled cover material on which there remains some dispute. For example, they contradict each other on the expanded/revised notation for the activity diagrams.