Try the GWT refcard at
GWT Style, Configuration and JSNI Reference | Refcardz, it's a nice free cheat sheet to help you learn and use GWT by Robert Hanson.
We also have a 50% off Manning books for DZone members this week, if you want to check out the GWT in Action book. Just use coupon code refcardz50 when you check out at
Manning Publications Co. and I think it's valid until Feb 3rd.