• Official GTK documentation:
  • All GTK libraries: includes GTK 4 and GTK 3.
  • The Widget Gallery, to visually choose a widget. You can also launch the gtk4-widget-factory application (gtk-4-examples package) to see and try all the GTK widgets in one window.
  • GNOME Developer Documentation.
  • Drawing with Cairo and GdkPixbuf:
    • Cairo documentation
    • GdkPixbuf
• The official GTK Discourse Forum, to get help about GTK.
• (fr) Forum GTK de developpez.com
• The GTK blog.
• GTK on Mastodon.
• The GTK Project.
• This Week in GNOME: "A short, weekly summary in which maintainers briefly announce what they worked on for the past week."
• Cambalache, the new RAD tool for GTK 4 is under active development.
• Blueprint, a markup language and compiler for GTK 4 user interfaces. It compiles to GtkBuilder XML.

Books about GTK are rare. There is especially the Andrew Krause's books: the first one is about GTK 2 (excellent but now obsolete, oriented toward the C language), the second one is the GTK 3 edition but for Python:

• Andrew Krause, Foundations of GTK+ Development, Apress, 2007, ISBN-13: 978-1-59059-793-4, 630pp, $36.99 (eBook). You can download the C examples here. And you can have a look at the book on Google Books.
• David Ashley, Andrew Krause, Foundations of PyGTK Development - GUI Creation with Python, Apress, 2019, ISBN-13: 978-1-4842-4178-3, 563pp, $34.31. You can download the Python examples here. And you can have a look at the book on Google Books.