Between Earth and Arcturus by James Prescott
“Between Earth and Arcturus is a 28th century adventure following Commander José Aguire of the Exploration Corps, and Lynda Stokes, an heiress who has a bad temper and superhuman strength, as their Captain’s mind slowly becomes unhinged.
The Earth government has been taken over by a ruthless, evil power, and the Captain conspires to start a rebellion. The one thing nobody counted on was running into an alien civilization, which becomes a game-changer, and further pushes the captain’s sanity towards the brink.
The alien king needs contact with Earth, but the political opposition in the alien government will stop at nothing to ruin his plans and take control. Commander Aguire and Lynda Stokes are the last hope for both the aliens and the suffering people of Earth.” (from the page at Amazon.com)