Found this book online, only one that was available for viewing for free so maybe it's worth a shot? http://www.scribd.com/doc/35089318/Object-Making-With-ArchiCAD
Just check out youtube as they have quite a few video's or tutorials on creating objects. Good luck!
A really good book is the GDL Handbook: a comprehensive guide to creating powerful ArchiCAD objects by Andrew Watson, this one helped me a lot.