From publisher blurb:
Hermetic Magic can be labeled (and in many circles it is) as one of the most complicated systems of spiritual attainment. Hermetic Magic has sometimes been termed "Ceremonial Magic" because Hermeticists tend to use complex Ritual and symbolism.
Hermetics are Seekers; Seekers of Divine Truth, of Wisdom, of Understanding, of Gnosis. This curiosity may be partially satisfied through books. As a literate and literary tradition, Hermeticism holds that as seekers we can benefit through the recorded experiences and insights of others; as well as through the mediation of a living teacher and/or a group of fellow seekers.
This supplement for D20 Modern describes Hermetic (or Ceremonial) Mages. Most members of secret societies such as the Golden Dawn, the OTO and such would be classified as Hermetic Mages. It provides a wealth of information, not only in crunchy stats, but there are 6 pages alone detailing what it means to be Hermetic.
Arcane Class - Hermetic Mage can also be used with Magic: The Science and Art of Causing Change (another D20 Modern supplement from 93 Games Studio).