Christian literature in Middle Persian

Now here’s a thought:

The century again witnessed several periods of open warfare between the two empires, but by this time the Sasanian authorities no longer felt any serious need to doubt the loyalty of their Christian subjects. It is significant that the synod of 576 instructed that prayers for Khosroes I always be included in the liturgy. Worth mentioning too in this context is the fact that the sixth century also saw the growth of a Christian literature in Middle Persian (whose existence is only known indirectly today). — Sebastian Brock, The church of the East in the Sasanian Empire up to the sixth century, in Fire from Heaven (1988)p.77)

I don’t know a sausage about Middle Persian literature.  Indeed my only knowledge of it is of a introductory treatise on Aristotle by Paul the Persian, which Severus Sebokht translated into Syriac. 

I wonder if any of this literature exists today?  Sadly Brock gave no footnote.