The recommended documentation is the tutorial/manual which is available on the wiki site here. You can also find an older printable version of the manual here.
Andrea Wiggins has also provided a great, easy-to-follow, tutorial for getting GUESS working on Mac OS here.
If you're interested in the Javadoc you can browse that here. If you get stuck you should also browse through the discussions on the mailing list.
GUESS maintained by Eytan Adar, Website update: 08/13/2007.