Complex sentence: When it comes to the'Catalog of all catalogs that don't list themselves', the librarian cannot include it in its own listing, because then it would belong in the other catalog, that of catalogs that do include themselves.
Split into simpler sentences: When it comes to the'Catalog of all catalogs that don't list themselves', the librarian cannot include it in its own listing, because then it would include itself. But in that case, it should belong to the "other" catalog, that of catalogs that do include themselves.