What is the most energy efficient way to heat a converted garage space?
In that climate, the most energy efficient method is a heat pump.  An added bonus is that it can air condition during hot times, something that is presumably a requirement for an office space or guest room in SoCal.

Heat pumps come in all kinds of flavors, but those designed for and often used in commercial and residential space should be plenty quiet.  Perhaps a 3/4 ton (9,000 BTU/hr) ductless unit (at $500) like this would be more than adequate:



Beyond that, energy efficiency can be further increased with better insulation, decreased air infiltration, and more efficient windows.

Comfort probably can be further improved by adding thermal mass:  heavy furnishings, dense partitions, heavy curtains and wall hangings, etc.