References: Unified Modeling Language (UML)

Articles and Books: There are several good books and articles on UML, including:

Alhir, S.S. (1998) UML in a Nutshell, O'Reilly and Associates , Sebastopol, CA.
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Booch, G. , J. Rumbaugh and I. Jacobson (1999) The Unified Modeling Language User Guide, Addison Wesley , Reading, MA.
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Muller, P.-A. (1997) Instant UML, Wrox Press, Ltd. , Birmingham, UK.
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Tutorials/User Guides: There are several WWW pages that can help you learn UML and use UML tools, including:

Embarcadero Developer Network UML Tutorial
IBM UML Tutorial
SPARX Systems UML Tutorial
Visual Paradigm for UML User's Guide

UML Reference cards: There are several UML "reference cards" available, including:

Provided by Holub and Associates
Provided by Visual Paradigm

Organizations: There are several organizations devoted to UML, including:

Object Management Group