I'm the author of the libGimbal project linked to there. It's a massive object-oriented C codebase with an entire cross-language type system that was inspired by GTk's GObject, Microsoft COM, and the Objective-C language.

I've studied everything out there on achieving OOP in C, and actually the OP's book reference was my original inspiration. There's a fine art to doing it and a few good codebases are out there that do it well... The entire GNOME stack is based on this kind of stuff.

Anyway let me know if you have any questions. lol.