Book probably from 1970s to very early 80s. Human-like "aliens" had already arrived and established themselves on Earth by selling advanced medical and longevity treatments.
A secret organization attempts to infiltrate them and an agent discovers that the "aliens" are actually from a parallel Earth. The advanced medical technology was coming from another parallel Earth (medical planet). The "aliens" had invaded a number of parallel Earths and were exploiting the resources of all of them.
I also seem to remember something about a strong caste system among the invaders.
Any help would be appreciated. Thanks.
A secret organization attempts to infiltrate them and an agent discovers that the "aliens" are actually from a parallel Earth. The advanced medical technology was coming from another parallel Earth (medical planet). The "aliens" had invaded a number of parallel Earths and were exploiting the resources of all of them.
I also seem to remember something about a strong caste system among the invaders.
Any help would be appreciated. Thanks.
