Jonathan Jongsma has recently pushed, pulled, and tweaked the gtkmm manual a bit. He’s added extra semantic markup and created a stylesheet to present it properly. It’s a big improvement on the old non-styled version. For instance, the view part of the gtkmm treeview section.
Jonathan Jongsma is the kind of person that I’d like to encourage to get more and more involved. He’s also doing lots of work on the Cairo C++ bindings (cairomm) and has already written the Cairo chapter in the gtkmm book.