Steuler in Germany has been one of the leaders in the technology, at least from my experience, but others like Johnson-Mathey, Miratec and DCL all have provided systems for quite some time and just about all the suppliers have good technical papers on the subject. CARB also has published a number of articles on SCR's for stationary and mobile sources.
It has become a pretty broad topic, are you interested in SCR's as applied to stationary or mobile sources? What kind of prime mover?
Probably your best overall source of info is a Google search narrowed by what ever specific info you're looking for.