There is not much choice. If you want to use free GIS software and you are starting out, you'll pretty much need to buy this book to get an overview of the possibilities.
Not that it's perfect, the book is necessarily a little outdated now, since the versions of such software such as Quantum GIS are no longer version 0.9 but 1.02 for the stable version and 1.3 for the development version.
But heck it's not the bells and whistles that count, it's the overall picture so you know where to look.
This covers a lot of different FOSS GIS applications, like GRASS UDIG, QGIS,OpenJump and others, but it gives most attention to GGis and Grass, which is not surprising, given the author is the person who started QGis and it works with Grass.
I would have saved a month of time had I started with this book rather than pester people on forums.