The Romulan Stratagem by Robert Greenberger

On a mission to an unexplored planet near the Klingon/Romulan border, the "U.S.S. Enterprise"(TM) travels to the planet Eloh to negotiate Federation memebership. But upon arrival, Captain Jean-Luc Picard and his diplomatic team find themselves face to face with Commander Sela, an old Romulan enemy -- who's there to convince the Elohsians to join the Romulan Empire. When a series of fatal indidents casts suspicion on both delegations, Data must form an uneasy alliance with Sela to prove the innocence of the "Starship Enterprise"(TM) crew -- or lose a sprategic stronghold to the Federation's greatest enemy.