David Nicholson-Cole's GDL Cookbook is definitely the place to go after you have mastered everything in the object-making book (which he also wrote) that comes with ArchiCAD 9.
I was hesitant to mention it because it has been out of print for a year while he has been working on version 4 of the book (publication date still not announced).
I see now that his website mentioned that he is selling the PDF version of the Cookbook 3.01 in the interim. Certainly anyone trying to learn GDL should obtain a copy.
😉
(Personally, I use the PDF version more than the hard copy anyway, since the search function in Adobe Reader will find many more things than are listed in the index. An added plus is that the PDF is in color.)
See:
http://www.nottingham.ac.uk/sbe/cookbook/gdl_cookbook/index.html
and email David directly at one of the addresses listed at:
http://www.nottingham.ac.uk/sbe/cookbook/
to purchase the PDF.
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB