Commentary by
critics and
reviewers;
literature (in the
scientific and
literary senses) written not about the
real world but about other
literature.
Literary criticism was once secondary literature, but has become a commentary on itself, like a snake head eating the head on the opposite side.
This is not to say that secondary literature is a bad thing. In the form of magazines like Discover and Scientific American, synopses of research make science more accessible to the public. It's simply a distinction between writing and writing about writing.
Compare primary literature.