The Elements of UML Style …
The Elements of UML Style, Scott W. Ambler (Cambridge) is one of the small pocket books like The Elements of Java Style that is very handy to have around. I’m constantly forgetting little bits here and there about UML like if dimonds are filled or not-filled and this reference covers everything in a handy keep on your desk or in your backpack size. There is even a section on Agile Modeling.