Geothermal energy is comprised of the energy remaining within Earth from its formation as well as the thermal energy that has resulted from the radioactive decay of atoms. Many know it's power from having observed hot springs, but this energy can also be harnessed to generate electrical power.
The United States in particular is responsible for nearly 30% of global geothermal energy production, creating domestic opportunities in this field. Geothermal energy engineering is based on the principles of geotechnical and mechanical engineering, and owes a great deal to a sound understanding of geological formations in the Earth, specifically hydrothermal vent systems.
Geothermal Energy Science (A scientific open-access journal)
US Department of Energy (Good links here)
National Renewable Energy Laboratory (More good links, more technical)
National Resources Defense Council (Well-cited sources to follow for more detail)
MIT Report on Future of Geothermal Energy (Older source, but still useful)