Second Stage Lensmen
Kim Kinnison was Number One of his time. To him fell the incredible assignment of infiltrating the inner circle of Boskone. Kinnison's deadly job was to become a loyal Boskonion in every gesture, thought, and deed. He had to work himself up through the ranks of an enemy alien organization, into the highest echelons of power - until it was he who would be issuing the orders that would destroy his own civilization!
Back to my teenage years
Loved this series back in the late 60s but many of the attitudes have not stood the test of time. However it remains a good romp of a story