I'm surprised that no one has reviewed Time's Enemy, because it might well be the best Star Dreck novel ever written. Even better than my favorite, Chain of Attack.
A starship, nearly torn to pieces, is found hidden in a comet. After study, Starfleet finds it to be an alternate version of the Defiant...nearly five thousand years old. The Deep Space Nine crew (most of them) are brought in to solve the mystery of how their ship was thrown into the distant past and wrecked in battle.