Having read the first few chapters of the book, I can already tell the purchase has been a good investment. The material is well structured and enjoyable to read (like all good books should be!). Another good thing about the book is the careful use of source code listings. Some other books just stuff page after page with source code listings which does nothing more than increase the page count. The source code listing in this book is brief and to the point.
The book has good coverage on GDK and GLib. It is divided into two main sections, a user guide (~700 pages) and reference (~100). The small appendix on Motif is useful for us old Xt/Motif timers. I was able to quickly grasp the key differences between both programming models.
The one thing that should have been left out is the "C Programming Review" appendix (~40 pages). If you don't know C yet then you shouldn't be reading this book anyway.
The poster is a nice touch. There is no CD-ROM but you can download most of the code from the web. I say most because week 3 of the source code is not yet available but will soon. I am willing to overlook this for now since the book has just been published.