The following resources are useful for users of The GIMP, even if some of them apply to older versions.
provides access to the internal help system. This documentation is also available in HTML and PDF formats at http://docs.gimp.org.
Find many tutorials explaining basic or advanced image manipulation techniques with The GIMP at http://gimp.org/tutorials/.
The GIMP User Group offers an informative Web site at http://gug.sunsite.dk.
http://www.gimp.org is the official home page of The GIMP.
Grokking the GIMP by Carey Bunks is an excellent book based on an older version of The GIMP. Although some aspects of the program have changed, it can provide excellent guidance for image manipulation. An online version is available at http://gug.sunsite.dk/docs/Grokking-the-GIMP-v1.0/.