There are many good books and sites on UML, and these lists are a short sample of them. Also UML is evolving, and the books can go out of date, however, the core part of the UML specification is stable and is not expected to change dramatically. Also, there are many online resources.
[umlspec-1.5] Object Management Group, OMG Unified Modeling Language Specification, Version 1.5 (OMG, 2003-03-01)
A reference document, with examples. Can be downloaded from the The Rational web site.
[umlug] Grady Booch, James Rumbaugh and Ivar Jacobson, The Unified Modeling Language User Guide (Addison-Wesley, 1999) ISBN 0-201-57168-4
A good solid reference and learning text.
[umldis] Martin Fowler and Kendall Scott, UML Distilled: Applying the Standard Object Modeling (Addison-Wesley, 1997) ISBN 0-201-32563-2
A good quick study. Short, with compelling examples, but should be combined with [umlug].
[omguml] Object Management Group, OMG Unified Modeling Language Specification, v1.4 (OMG, September 2001)
The official UML standard. Available at http://www.uml.org
[umlnut] Sinan Si Alhir, UML in a Nutshell (O'Reilly, 1998) ISBN 1-56592-448-7
A very condensed treatment of the topic, useful for reference purposes.
[ooad] Grady Booch, Object-Oriented Analysis and Design with Applications (Addison-Wesley, 1994) ISBN 0-8053-5340-2
Although dated in the second edition I used, had good material on O-O techniques. Diagrams need to be updated from Booch notation to UML.
[oocrc] Kent Beck, Ward Cunningham A Laboratory For Teaching Object-Oriented Thinking, http://c2.com/doc/oopsla89/paper.html
The original paper describing the CRC card method, as presented at the OOPSLA 1989 conference (Object-Oriented Programming, Systems, Languages, and Applications). A local copy is also preserved at Ralph's web site.
[omg] Object Management Group, UML Home Page, http://www.uml.org
The standards organization that maintains the specification of UML.
[rational] Rational Software's Rational Home Page, http://www.rational.com
A primary vendor of UML compatible modeling tools, and also a good source of online reference materials for UML.
$Author: rlv $ - $Date: 2004-03-20 18:20:03 $ $Name: not supported by cvs2svn $ - $Revision: 1.11 $ $Source: /h/cm/uml/refs.html,v $Ralph L. Vinciguerra