In the tradition of the old "Ace Doubles" two-in-one books (flip one over to read the second title) -- here is the fifth Wildside Double: the two-volume "War Surplus" series by Lawrence Watt-Evans, "The Cyborg and the Sorcerers" and "The Wizard and the War Machine."
THE CYBORG AND THE SORCERERS
The cyborg code-named "Slant" was sent out as an Independent Reconnaissance Unit during an interstellar war between Earth and its colonies. The fighting ended three hundred years ago, but Slant's computer does not admit this -- he is compelled to carry on as if the war were still raging.
Then he comes across a planet where his sensors register "gravitational anomalies." The computer interprets these as enemy weapons research.
The local inhabitants call the anomalies "magic."
THE WIZARD AND THE WAR MACHINE
At the end of The Cyborg and the Sorcerers, Sam Turner was making a life for himself on the planet Dest. He thought he had left the long-lost interstellar war between Earth and its rebellious colonies behind him forever.
"Forever" turned out to be eleven years. That was how long it took for another Independent Reconnaissance Unit to respond to the distress call his ship had sent before it was destroyed.
And this one made his own berserk killer computer look sane.