SWT was written to support the Eclipse IDE. Eclipse is at http://www.eclipse.org/.
An introduction to SWT can be found at the same site: http://www.eclipse.org/articles/Article-SWT-Design-1/SWT-Design-1.html.
Part 2 of the same article is at http://www.eclipse.org/articles/swt-design-2/swt-design-2.html.
The full list of Eclipse technical articles (including those on SWT) may be found at http://www.eclipse.org/articles/index.html.
A good introductory article can be found on the Developer.com Web site: http://www.developer.com/java/other/article.php/3330861.
As always, consider using Google.com to find additional information.
In dead-tree form, Chapter 10 of the book The Java Developer's Guide to Eclipse by Sherry Shavor et al. (Addison-Wesley, ISBN 0-321-15964-0), also provides an introduction to SWT.
