Online book about "Learning Advanced Image Editing Techniques" with the Gimp. From the preface:

The most annoying thing was that almost every book I picked up was full of tips and tricks. Tips and tricks? I felt like I was getting advice on betting the ponies. I didn't want tricks; I wanted the ideas. What is photo touchup and enhancement? Where's the beef? How could I work on my photos if I didn't understand the basic concepts?

Working with digital images requires some understanding of what needs to be done. It's not a ``choose the right tool, one click, and you're done'' subject. Most books on digital image manipulation would have you believe the contrary...and perhaps they're right. However, that is not what this book is about. It is not about tips and tricks, and it's not a collection of recipes for solving someone's favorite image manipulation problems. It is first about understanding image manipulation concepts, second about knowing which GIMP tools are most effective, and third about the savvy use of these tools.

See also Photo Touch Up and Enhancement in the GIMP by the same author.