The geoCable is a custom cable that interfaces the user port on the Commodore 64 to the parallel port on a printer.  I found this short article on how to make the cable HERE. Thank you Internet Archive for preserving this!  

Further information on this topic can be found HERE , HERE , HERE and HERE.  I've never built one of these cables, so I recommend cross referencing all the information until you're comfortable with your chosen solution.

Also, issue 72 of geoNews (August 1997) has a short article on page 3 about creating this cable.  

One last source for info on the geoCable - the CMB FAQ v3.1 - http://www.faqs.org/faqs/commodore/main-faq/part7/

geoFont Catalog is a listing of almost all the fonts available for GEOS printed in the actual font of each respective type.  This is a worthy document to have printed and near at hand if you do much with GEOS.  The document originates from https://www.lyonlabs.org/commodore/onrequest/geos/geoFont.pdf

This PDF is an excellent tutorial on how to use geoPublish to create a newsletter.  The tutorial is very thorough and full of great hints and tips.  Originally the source of this document was http://members.shaw.ca/cue64/geopindex.html but unfortunately that site no longer exists.

Some good programming information for the GEOS desktop replacement called Wheels.