To start off, I would recommend the "Object Making with ArchiCAD". It was written to use with AC 6.5/7.0 but most of the basics of GDL haven't changed much since then. It's a good start for beginners.
If you are planning on diving into GDL head-first I would highly recommend the "GDL Cookbook" (If you can find a copy). It is a very useful resource for anyone working in GDL.
David Larrew, AIA, GDLA, GSRC
Architectural Technology Specialist
a r c h i S O L U T I O N S
WIN7-10/ OSX 10.15.7
AC 5.1-25 USA