Simon and Schuster website has been updated with a new listing for another Typhon Pact novel: Brinkmanship, by Una McCormack. It's due out in September (or in other words is the October book). Which in turn means David Mack's TNG trilogy presumably won't be starting until at least November.
McCormack's previous Trek works include Cardassia: The Lotus Flower (in the Worlds of DS9 series), Hollow Men, and the brilliant, The Never-Ending Sacrifice. So Cardassian-centric Typhon Pact novel maybe?