The Old GDL Reference Book is on-line

The last major reference work devoted to GDL (Geometric Description Language) was David Nicholson-Cole's seminal GDL Cookbook.

Cookbook 3.1 apperared in print form in 2001 and is available today as a PDF. An interactive Cookbook - Cookbook 3.99 - went on-line in 2003. All of this circa ArchiCAD 7.0.

This was the book that made sense of GDL, taught us structured programming practices and refresed us on BASIC (which is kinda what GDL is) and made parametric object scripting fun.