The theory of group schemes seems to be well developed: there are many applications and examples, and the literature is vast.

On the other hand, a quick google search with some obvious keywords (ring, schemes, algebraic) does not yield any good reference on Ring Schemes, nor any interesting examples (other than Witt schemes) or applications. My question, out of pure curiosity, is:

Is there any good introductory reference(s) to the subject?

I'd be most interested in knowing what the important questions and applications of Ring schemes are.