Reliable EDM has a series of videos available on Electrical Discharge Machining or EDM. The company president, Carl Sommer, presents this video that introduces the three types of EDM: wire EDM; ram/sinker EDM; and small hole EDM.
The most interesting part of the video is the skim cut being made by wire EDM out of the bath. You can see the sparks being created across the dielectric fluid that erode the material.
Carl mentions a book on Electrical Discharge Machining that you can get from his company. You can find an electronic edition of his book, Complete EDM Handbook, by following this link. If you would like a hardcopy of the book it is available on Amazon.