Old Church Slavonic

[Quasus]

I'd like to learn something about Old Church Slavonic.

As far as I am aware, Old Church Slavonic (Russ. старославянский язык) is a literary language created by Constantine (Cyrill) on the basis of vernacular Slavic dialects in order to translate religious literature for the use of the Slavs. As such it possessed a limited vocabulary and could hardly be used for non-religion purposes. Correct me please if I'm mistaken.

Later on it evolved into different local variants of "Church Slavonic" (изводы церковнославянского). Could anyone tell to what extent Church Slavonic was in use, say, during the middle ages? Is there any secular literature in it? Could anyone share a link to an online library if there exists one? And finally, how significant are the differences between its local variants siue изводы?