The documentation that is currently available on this site is listed below.
Command line options The possible command line options to mkgmap. This list is also available using the command:
Style Manual [pdf] This is the complete documentation of the style rules that determine how the OSM taged features are converted into the Garmin features.
TYP compiler manual This documents the language that is accepted by the TYP compiler that is included within mkgmap.
For other documentation go to the Open Street Map wiki for documentation here