In my experience, the phrase harsh-environment really doesn't well define any environment at all. The environment experienced in space exploration is a world apart from that of the insides of an engine bay again much different to the harsh environment within a few metres of a nuclear reactor core.
Its a matter that you really have to boil down to the particular environmental parameter you're interested in such as vibration, ionizing- and nonionizing-radiation, temperature and humidity etc and look at the design criteria for such matters. I work primarily with electronics in some pretty horrible conditions with regard to corrosive atmospheres, ionizing radiations and extreme temperature cycling, the only thing I don't have is vibration going on. I never managed to find a reliable text on the matter, but there are a lot of incidental application notes from various suppliers targeting some components for 'harsh' environments.
It would be worth looking up "design for high temperature", "design for high vibration",... etc