Dark Mirror

Hardcover ISBN 0671793772 written before DS9 aired the mirror universe episodes. The Enterprise-D crew is drawn into another dimension and learns that it's the mirror universe first visited by Kirk and three other crewmembers a century earlier. They learn the Empire has found a way to cross the dimensional barrier and intends to invade the Federation and Romulan and Klingon empires, and must find a way to thwart this attempt.