GIMP 2.8 for Photographers: Image Editing with Open Source Software
GIMP for Absolute Beginners
The Artist's Guide to GIMP: Creative Techniques for Photographers, Artists, and Designers (Covers GIMP 2.8)
Sams Teach Yourself Inkscape, Gimp and Blender in 24 Hours (Sams Teach Yourself...in 24 Hours (Paperback))
Instant GIMP Starter