The Wounded Sky (Star Trek: The Original Series, #13)

An alien scientist invents the Intergalactic Inversion Drive, an engine system that transcends warp drive — and the U.S.S 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!