This book is based on GIMP 2.4. But software projects don't stand still, and as the second edition goes to press, GIMP 2.5 is being built into what will eventually become the GIMP 2.6 release.
Although there's no telling what else might change, here is a preview of some of the important differences you're likely to see in GIMP 2.6. You can get a look at them yourself by downloading one of the experimental 2.5 builds (but be warned: not all the bugs have been squashed yet). Among the important differences are
No more Toolbox menus (and the new "no image" window)
The improved Free/Polygonal Select (Lasso Select) tool
GEGL (the Generic Graphics Library)
Color tool improvements
"Brush Dynamics" for painting
Text tool wrapping ...