How many texts are comprised in the Mimamsa Sastra? And why is it relevant?

(apologies in advance for the lack of diacritics, I am home, ill, with no access to a unicode keyboard)

Purva Mimamsa authors are generally not interested in the topic, whereas several Uttara Mimamsa (i.e. Vedanta) ones deal at length with the status of the Mimamsasastra (I am tempted to say that, similarly, Christians alone are concerned with the unity of the two testaments within the Bible).
A particularly puzzling element, in this connection, is the status of an “intermediate part” of the Mimamsasastra,

Scripture, authority and reason —About a new book edited by Vincent Eltschinger and Helmut Krasser

How do reason and authority interact and trace each other’s boundaries? Which one is the first to be allowed to delimit its territory and, by means of that, also the other one’s one?