|Getting Started with The GIMP: an informal tutorial: CSLUG and Rock Eagle Presentations, October 2002|
The tutorials at the GIMP site are very good: http://www.gimp.org/tutorials.html
There are at least two books available online: the GIMP User's Manual, aka GIMP: The Official Handbook available at http://manual.gimp.org/, and Grokking the GIMP available at http://gimp-savvy.com/BOOK/index.html. Of the two, I definitely favour the latter: it's fairly straight-forward, while I find the former difficult to use online and sometimes overly complex. Both are also available in paper versions: I think I'd like the User Manual better in paper, but I don't own it and can't say for sure.
Two other good books on the subject are Alex Harford's GIMP: Essential Reference, published in 2000 by New Riders, and Michael Hammel's Essential GIMP for Web Professionals published in 2001 by Prentice Hall PTR. Of all of these books, I find myself referring to GIMP: Essential Reference the most. I'm not sure it's the best to learn from (Essential GIMP for Web Professionals was pretty good for that), but once you know the GIMP it's very well organized as a reference (I don't take it for granted these days that something that says it's a "Reference" will actually be good for that ...)
The GIMP plugin registry at http://registry.gimp.org/.