Michael Hammel was a regular contributor to Linux Format magazine and released
several of his GIMP tutorials as PDFs -- these examples will give you an idea as to the quality of his writing skills (which, for what it's worth, I consider to be very good), and you just might learn something as well

.
You might also considering saving your money for other pursuits* (such as
making a donation to the GIMP project) and focusing on many of the free online options for learning to use GIMP on your own. There are several online tutorials -- both video and textual -- as well as the original
Grokking the GIMP, which is a bit outdated with regard to the GIMP interface but the image manipulation techniques presented are still valid.
You could even reconcile yourself to learning from, and asking questions of, the members of the many GIMP users forums such as this one.
* I am a great fan of "dead tree" books (some might say I am a voracious reader), and am certainly not opposed to supporting authors who write about GIMP. Nonetheless, I personally prefer to learn about computer things while sitting in front of a computer.