The guide is targeted at the 1.2 release of the GIMP but was wirtten before 1.2 was actually released; what this means is that, while the guide is about 95% correct, there are portions of it that are no longer exactly relevant and there are other features of the released version of the GIMP that are either documented incompletely, incorrectly or not at all.
However, for somebody who already knows the generalities of the GIMP this is an excellent occasional "how did that feature work again?" quick-ref guide.
Of course, what do you expect from an O'Reilly book?