Here are a few examples of some ArchiCAD theory from On Land (the on-line ArchiCAD manual for Rill Architects in Bethesda, MD). James Murray, the guy who maintains this site for Rill, is one of the ArchiCAD gurus for Jared Banks, aka Shoegnome, who is one of the pre-eminent ArchiCAD gurus out there right now, and certainly one of my ArchiCAD Heroes.
Note that these articles were all written nearly a decade ago, so there will be a few items mentioned in these articles that will be outdated, issues that have been fixed or have been rendered non-issues by features that have been added in subsequent versions of ArchiCAD. But MOST of what James has written here is still relevant and is super informative.
A Strong Model Can Redeem a Weak Drawing
Beauty is Third
Degrees of Difficulty
And from a different source, from bd-MAP, the on-line ArchiCAD manual for Bushman Dreyfus Architects in Charlottesville, VA, here is another mantra:
These are very short articles. Take 10 minutes and read them. Then read them tomorrow morning. Then, come back to them in a week, or a month, or sometime later. To touch base. To take a minute to contemplate how you (and we) are using ArchiCAD.