Where to Learn More
Along with the popularity of the UML has come a wave of books teaching its techniques and principles. If you want to learn more about the UML, you may want to start with the book UML Distilled, Second Edition by Martin Fowler. It gives a good overview of both the Unified Modeling Language and the Unified Process (a software development process that ties in nicely with the UML). It's a small book and an easy weekend read.

If you're looking for books direct from the creators of the UML, then you should check out the following three books from Booch, Jacobson, and Rumbaugh (all from Addison-Wesley):


Because use case diagrams are such an important part of the UML (some would argue, the very foundation of the UML), you may want to read a book devoted entirely to use cases. The book, Object-Oriented Software Engineering by Ivar Jacobson, provides a deeper understanding of use cases than you'll find in any of the books mentioned above.