Engines of Destiny - Star Trek Novel

THE MACHINE AGE
After the apparent death of James T. Kirk on the U.S.S. Enterpris NCC-1701-B, Montgomery Scott decides to leave Starfleet. A chance encounter with a mysterious woman named Guinan leads him to hitch a ride on the U.S.S. Jenolen -- which crashes into a Dyson Sphere, leaving only Scotty to survive suspended as a transporter pattern....
Seventy-five years later, he is revived aboard the Enterprise-D -- on which Guinan serves as bartender. But fate has not finished with either Scotty or Guinan. The engineer hatches an audacious plan to travel back in time and rescue Jim Kirk the moment before he would vanish into the nexus and bring him back to the twenty-fourth century without damaging the timeline.
Scotty's plan, however, does not go as expected. Though Kirk is rescued, the famed captain and the engineer, as well as Captain Picard, Guinan, and the crew of the Enterprise-D, are trapped in an altered past where there is no Federation, and no human race. Sarek of Vulcan leads a ragtag collection of species fighting a losing battle against the Borg, who have swarmed into the Alpha Quadrant and assimilated Earth.
Starfleet's finest miracle worker must find a way to restore the timeline before the galaxy is overrun....