I have been using GIMP, initially 2.6 and now 2.8, for some time for processing and retouching my photos. I bought this book in Jan 2011 so I have been using it on and off for a couple of years. I see that an update for 2.8 by the same author is due in May 2013, so I thought a review would be timely.
The book is very comprehensive and detailed. It is organised as a series of exercises (with the necessary sample images and other resources available from the included DVD). The material is well and clearly explained and the exercises are covered in a clear, step by step approach. They tend to start off with very detailed hand-holding and then the hand-holding is withdrawn as things progress and the instructions become somewhat terser. I rather like this approach - it avoids becoming patronising which is one of the pitfalls of this sort of detailed step by step approach.
Its subtitle is "Image editing with Open Source Software" and it does indeed cover several other pieces of software in some detail, particularly the raw converters UFRaw and RawTherapee. The DVD includes copies of GIMP and the other pieces of Open Source software mentioned.
It starts off with the basics of loading and saving images, cropping, resizing and various basic touching up: colour correction, removing spots and scratches, sharpening, etc. There is extensive coverage of working with scanned images, which I find particularly useful as I have an extensive collection of 35mm slides and need to deal with these from time to time when organising materials for talks.
A number of common filters are covered in some detail for things like red-eye removal, perspective and lens distortion correction, frames and vignetting and dealing with under- and over-exposure. It then moves on to layers and masks with a series of exercises covering things like replacing the sky in an image, removing increasingly more complex objects from their background and building composite images. I have found these exercises very useful and have returned to them a number of time when I need to remind myself how to do some of the more complex tasks.
There is a brief appendix "A Forecast on GIMP 2.8", but this book was written a couple of years before the release of GIMP 2.8 and cannot therefore say much about the changes. The user interface of GIMP 2.8 has changed somewhat (especially the single window mode and the major new "Gotcha" of separate Save and Export commands in the File menu) but, in general, the exercises work perfectly well in GIMP 2.8.4 (the current version at the time of writing). I will be interested to see the updated version of the book when it arrives!
Overall, I would thoroughly recommend this book both to the new comer to GIMP and to the more experienced GIMPer who wants to remind themselves about some of the more complex stuff.