Where is the documentation for GTK+? [GTK 2.x]

In the GTK+ distribution's doc/ directory you will find the API Reference for both GTK and GDK, this FAQ and the GTK Tutorial.

In addition, you can find links to HTML versions of these documents by going to http://www.gtk.org/. A packaged version of the GTK Tutorial, with SGML, HTML, Postscript, DVI and text versions can be found in ftp://ftp.gtk.org/pub/gtk/tutorial

There are now a few books available that deal with programming GTK+, GDK and GNOME. Unfortunately, they currently are all based upon GTK+ 1.x: