David Mack: Precipice (Star Trek Vanguard) [2009] paperback

Diego Reyes, courtmartialled Starfleet commodore and disgraced former commander of space station Vanguard, discovers that his role in deciphering the truth about the Taurus Reach is not yet over. As friend and foe join forces in a separate peace against the threat of the Shedai -- the godlike aliens who, eons ago, reigned over that part of the galaxy -- a long-missing wild card in the complex events which are playing out in the Taurus Reach returns to change the nature of the game.