Modeling and the Unified Modeling Language

Booch, G., J. Rumbaugh, and I. Jacobson. 1998. The Unified Modeling Language . Documentation set, version 1.3. Cupertino, CA: Rational Software.
The latest OMG standard on the UML at the time of this writing. Available online at www.rational.com.

Booch, Grady, James Rumbaugh, and Ivar Jacobson. 1999. The Unified Modeling Language User Guide . Reading, MA: Addison-Wesley.
Published at the same time as the Rational Unified Process 5.0, this manual is an excellent user's guide on UML by its main authors.

Eriksson, Hans-Erik, and Magnus Penker. 1997. UML Toolkit . New York: John Wiley & Sons.
A comprehensive book on UML as seen from Sweden by another pair of Rational friends .

Fowler, Martin. 1999. UML Distilled: Applying the Standard Object Modeling Language , Second Edition . Reading, MA: Addison-Wesley.
A very nice little introduction to UML if you are in a hurry.

Muller, Pierre-Alain. 1998. Instant UML . Chicago: Wrox, Inc.
Another short introduction to UML by a former colleague of ours.

Quatrani, Terry. 1998. Visual Modeling with Rational Rose and UML . Reading, MA: Addison-Wesley.
Provides step-by-step guidance on how to build UML models. At the same time, it follows the Rational Unified Process for which this book provides, in effect, a small-scale example.

Rumbaugh, James, Ivar Jacobson, and Grady Booch. 1999. The Unified Modeling Language Reference Manual . Reading, MA: Addison-Wesley.
Certainly more digestible than the OMG standard, this book is an in-depth examination of UML by its main authors.