There is a very good review article in the latest issue of Science that compiles all of our experimental and theoretical understanding of light propagation in a medium.
Abstract: It is now possible to exercise a high degree of control over the velocity at which light pulses pass through material media. This velocity, known as the group velocity, can be made to be very different from the speed of light in a vacuum c. Specifically, the group velocity of light can be made much smaller than c, greater than c, or even negative. We present a survey of methods for establishing extreme values of the group velocity, concentrating especially on methods that work in room-temperature solids. We also describe some applications of slow light.
 R.W. Boyd and D.J. Gauthier, Science v.326, p.1074 (2009).