Second stage Lensmen
From back cover of Jove/HBJ paperback October 1977: **interplanetary spy** Kim Kinnison was Number One Man of his time. To him fell the incredible assignment of infiltrating the inner circle of Boskone. Kinnison's deadly job was to become a Boskonian in every gesture, thought, and deed. He had to work himself up through the ranks of an alien enemy organization, into the highest echelons of power -- until it was he who would be issuing the orders that would destroy his own civilization!