There are some other great tutorials, guides, and books out there which cover advanced or specific topics. This chapter contains links to a number of them, but is by no means exhaustive.
Federico told me he would be writing up some documentation that I could link to from here. Someone should get on his case. :)
While these were already covered in the Important Websites section, I believe they also deserve mention here. The main GTK tutorial is found at http://www.gtk.org/tutorial. The GTKmm version (for the C++ bindings) is found at http://www.gtkmm.org/docs/gtkmm-2.4/docs/tutorial/html/index.html. The PyGTK version (for the Python bindings) is found at http://www.pygtk.org/pygtk2tutorial/index.html (though there is a newer version under development at http://www.moeraki.com/pygtktutorial/pygtk2tutorial/). Gtk2-Perl does not have its own version of the tutorial. I did find a preliminary one in CVS somewhere, but no longer remember where I found it. Note that the verions of the GTK tutorial for other language bindings often include a few more topics than what the original version has.
Language binding specific documentation
The documentation for C is (supposedly) kept at http://developer.gnome.org/doc/, though it has numerous problems to the point of being almost useless. GTKmm, PyGTK (see the "Documentation" box in the lower right corner), and Gtk2-Perl do much, much better at providing organized documentation.
This tutorial was written by Malcolm Tredinnick to cover the steps required to internationalize an application. While the tutorial provides examples for programs written in C, the principles and methods apply to other languages as well.
Matthias Warkus originally wrote this book in German, but it was later translated into English. The English version includes many updates and bugfixes, and has been thoroughly reviewed. This book is aimed at C programmers who want to know the deeper details. It was reviewed at OSnews.
This tutorial was written by Tim-Philipp Mueller to show how to use the GtkTreeView widget through its C interface. (Note that gtkmm includes coverage of GtkTreeView in its version of the Gtk tutorial, as does the newer version of the Python tutorial.)
Auto* build tools
Murray Cumming wrote a gentle introduction to automake and autoconf. While geared towards C++, most of the information is applicable for other languages as well. There is also a much more comprehensive book on Autoconf, Automake, and Libtool which has been made available online. Gnu also has comprehensive online manuals on autoconf, automake, libtool, m4, and gettext.
Written by Mathieu Lacage, this article covers low-level details of the Glib Object system. Most of the code in the article is in C, though there is some small discussion of other languages (after all, one of the main purposes of GObject was to make wrappers for other languages as automatic and straightforward as possible).