Application note (PDF!) on power gain stages for monolithic amplifiers from Linear:
The type of circuitry used in an output stage varies with the application, which can be quite diverse. Current and voltage boosting are common requirements, although both are often simultaneously required. Voltage gain stages are usually associated with the need for high voltage power supplies, but output stages which inherently generate such high voltages are an alternative.
A simple, easily used current booster is a good place to begin a study of power gain stages.