GTK+ is an open source GUI library, currently running on Linux although a Windows port is in progress. At nearly 1000 pages, this book looks more foreboding than its title, but after reading the first few chapters you can treat it as a reference, especially if you have a specific application in mind. It is not too difficult to locate specific information, but the book was not intended to be a comprehensive reference work.
There is room for improvement; there are typographical and occasional grammatical errors and sometimes errors in the listings and I would prefer to see a more structured approach with not so many hard-coded numbers and much in main() (modularisation is not introduced until day 7 and then is used sparingly). One of the appendices is a brief and not quite standards-conforming introduction to C, which I would not recommend (for example, the only method of input suggested is the dangerous gets function). There is a poster included showing the GTK+ class hierarchy but its layout could be improved.
The book is adequate if you are knowledgeable enough to avoid picking up bad practice, but there are some aspects of it that I would hope to be re-worked for the second edition.