The definitive guide to SWT and JFace / Rob Warner with Robert Harris. Apress Springer Verlag, c2004.
Here is a good introduction to Eclipse GUI programming. Though somewhat dated, it is still worth reading. It will help if you are doing a Rich Client (RCP) or extending Eclipse with a new plugin. SWT is way better than Swing, but there seem to be fewer books on it. There is a lot to learn, and too many sources on the net, so use this book to get started.