The Changeling Worlds — Kenneth Bulmer

On the gold-symbol world of Beresford's Planet, Richard Kirby lived in total luxury. As a member of "The Set" his life was a never-ending round of planetary party-hopping. The only restriction imposed on him - that he never put down on any world marked with a red or black symbol - was something that he had always accepted without question.

That is, until his brother Alec was murdered in cold blood! Alec had been an undercover agent to those forbidden planets, and in order to avenge him, Kirby had to find out for himself what was really happening there.

But with the start of his investigation, Kirby found out quickly that the authorities meant business when they said "Hands off!" The secret they were protecting was of vital importance, and it now became a matter of life and death, not only to Kirby, but to all the inhabitants of THE CHANGELING WORLDS.