The Wounded Sky — Diane Duane

A pretty alien scientist invents the Intergalactic Inversion Drive, an engine system that transcends warp drive--and the Enterprise will be the first to test it! The Klingons attempt to thwart the test, but a greater danger looms when strange symptoms surface among the crew--and time becomes meaningless.

Now Captain Kirk and his friends face their greatest challenge--to repair the fabric of the Universe before time is lost forever!