Counterpart theory in the SEP
Until recently, the Stanford Encyclopedia of Philosophy didn't have anything on counterpart theory. The editors thought the topic isn't worth an entry of its own, but at least it now has a section in the entry on "David Lewis's Metaphysics". This isn't ideal, since counterpart-theoretic approaches to intensional constructions are best seen as metaphysically non-committal. But it's better than nothing.
I also wrote an "appendix" to the entry with an overview over counterpart-theoretic interpretations of quantified modal logic. It explains some unusual features of counterpart-theoretic logics, how they arise, and how they could be avoided.