The regio marks the end of the tour of the caverns, and Gregorius brings you back up into the light - and then into the room in the main manor that houses the covenant's library.
"Houses" is about the best you can say of it - it's a small, dim room into which a number of chests have been squeezed. However, the catalogue which Gregorius shows you has a reasonable list of books on it, at least for a Spring covenant (https://forum.atlas-games.com/t/covenant/5282/1)
"Well, sodales," Gregorius smiles. "That concludes our tour, but do ask if you have any questions, either now or later. Either Sophia or our autocrat, Phocus, should be able to deal with any minor needs you have, but let me know of anything more substantial."