Perry's Planet

Summary
From the back cover: Planet of perilous peace

Kirk, Spock, Scotty and the Enterprise crew face battle with the Klingons in a strange world ruled by a "human" who's been dead for 300 years! Soon the entire crew has fallen under the planet's mysterious force -- stripped of their power to wage war -- as the subhuman Immunes advance... and the Klingon legions hover overhead!