How to classify the book  Agadot yam ha-Talmud al Masekhet Shabat by S. Vanunu, which is a collection of aggadic blurbs
from the usual cast of rabbis based on/related to passages in Masekhet Shabat.

The problem:  the commentary is Aggada, with the latter being a general subject heading. It's a quandry:  on the one hand, the book is Aggada (1st heading 650  Aggada--BM516), but on the other the Aggada is the commentary to the masekhet (1st heading 630  Talmud. Shabbat $v commentaries -- BM506).
Can the 6xx be Aggada if it is Aggada and not about Aggada?

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It seems that this work would require three subject headings in order to account for its being a commentary on aggadot that are contained in the tractate Shabbat. It seems best to give these three headings in the following order:

63000 Talmud.$pShabbat$vCommentaries.
650 0 Aggada.
650 0 Aggada$vCommentaries. [an established heading]

The appropriate classification would accordingly be BM506.S27.

As regards your question whether "Aggada" may be used for the aggadic texts themselves, that is indeed the appropriate usage. I gave it as the second subject heading to cover the portions of Aggada from tractate Shabbat that are contained in the work.  For criticism of Aggada, we would use either 650 0 Aggada$vCommentaries (as in this case, referring to the excerpts from rabbinic writings about the aggadot) or 650 0 Aggada$xHistory and criticism (for a work not in the format of a commentary).

The heading for the tractate seems better as the first subject heading, since the work is basically a commentary on the tractate by way of comment on the aggadot contained in it.

Peter Kearney