Thank you. We are encoding each sermon as a separate TEI document. Even though some of them are extracted from a larger collection (book) we do not have the cases of two or more sermons coming from the same collection. The goal is just to keep the track of the volume information such as the number of sermons within it and the thematic of the collection.
We do agree that the miss-use of existing tags is to be avoided, thus we were thinking of using <keywords> element you suggest.