Many fine books have been written and published on the subject of OO software development by a variety of publishers, and it would be virtually impossible to do justice to them all here. Consider this list to represent some of our personal recommendations (visit Jacquie's web site, http://objectstart.com, for more recommendations), but please do browse the titles available from your favorite technical bookseller because new titles are being released literally every day.
Booch, Grady, James Rumbaugh, and Ivar Jacobson. The Unified Modeling Language User Guide, Addison-Wesley, 1998.
A definitive reference on UML, written by its creators; definitely worth adding to your library if you're serious about object modeling.
Rumbaugh, James, Ivar Jacobson, and Grady Booch. The Unified Modeling Language Reference Manual, Addison-Wesley, 1999.
A second definitive reference by the same gentlemen; see our comments for the preceding title.
Jacobson, Ivar, Grady Booch, and James Rumbaugh. Unified Software Development Process, Addison-Wesley, 1999.
And a third!
Quatrani, Terry. Visual Modeling with Rational Rose and UML, Addison-Wesley, 1998.
A practical, step-by-step guide for how to use Rational Rose, one of the industry's leading object modeling Computer-Aided Software Engineering (CASE) tools, to prepare UML models.
Eriksson, Hans and Magnus Penker. UML Toolkit, John Wiley & Sons, Inc., 1998.
Shoemaker, Martin. UML Applied: A .NET Perspective, Apress, 2004.
Rosenburg, Doug and Stephens, Matt. Use Case Driven Object Modeling with UML Theory and Practice, Apress 2007.
Taylor, David A. Object Technology: A Manager's Guide, Addison-Wesley, 1998.
A classic, high-level review of the direction in which the OO industry as a whole is headed.
Gamma, Erich, Richard Helm, Ralph Johnson, and John Vlissides. Design Patterns: Elements of Reusable Object-Oriented Software, Addison-Wesley, 1994.
An in-depth look at identifying and reusing common design patterns.
Meyer, Bertrand. Object-Oriented Software Construction, Prentice Hall, 1988.
Provides an introduction to the C# language and then moves to a discussion of key technical and architectural issues for .NET developers.
Troelson, Andrew. Pro C# 2008 and the .NET 3.5 Platform, Fourth Edition, Apress, 2007.
Covers the Windows Forms namespaces as well as a detailed discussion of good user-interface design principles.
MacDonald, Matthew. Pro .NET 2.0 Windows Forms and Custom Controls in C#, Apress, 2005.
This list is by no means comprehensive. Check out the Apress website for other good C# or .NET references.