If the library you are interested in isn't listed here, try the documentation references page on developer.gnome.org where you will find many other APIs documented which are used throughout the GNOME platform. For documentation of the language bindings, you might be able to find it on the project's page, although the following documentation can still be of use.
GTK+ 3 with Builder
This book was written by Eike Lange (it's in German).
Foundations of GTK+ Development
This book was written by Andrew Krause.
The Official GNOME 2 Developer's Guide
This book was written by Matthias Warkus.
The Official Tutorial.
GTK+ 2.x Tree View (pdf)
This tutorial covers the GtkTreeView and was written by Tim-Philipp Müller.
GTK+ 2.x Text View
This tutorial covers the GtkTextView and was written by Vijay Kumar B.
GTK+ 2.x Tutorial for Beginners
This tutorial is for beginners describing some of the most common widgets written by Jan Bodnar.
Building GTK apps for MS Windows on Linux
This tutorial details information about cross-compilation and was written by Allin Cottrell.
Manage C data using the GLib collections
A look at using GLib to effectively manage data within C programs by Tom Copeland.
An article about input method modules and Pango engines by Theppitak Karoonboonyanan.
Highlights of GTK+ 2.10
Presentation by Kristian Rietveld and Tim Janik at GUADEC 2006.
The new GTK+ printing API
Presentation by Alexander Larsson and John Palmieri at GUADEC 2006.
Windows portability for GNOME software.
Presentation by Tor Lillqvist at FOSDEM 2006.