I wrote a now-thoroughly-obsolete book about GTK+ 1.2 and GNOME 1.4, called GTK+/GNOME Application Development. The book may still have the only good tutorial-style writeup of certain GTK+ internals, so you might find it useful; but many of the details have changed over the years.

The book is an "advanced internals"/"conceptual understanding" kind of thing, if you just want to know how to get a button on the screen there are better books to choose.

The old home page for the book, Amazon product page.