I remember this juvenile Sci-Fi novel I read sometime when I was a teenager; so it probably was written in the 1960s, or possibly in the early 1970s.
One of the central characters is an adolescent boy who was either stranded or shipwrecked on a planet, and he has been alone all of his life except for a voice he hears in his head. It turns out this voice belongs to an alien creature who has been guiding and mentoring this boy all his life. They essentially have a telepathic connection.
Meanwhile, across the galaxy on another planet, there is an adolescent girl who is also telepathic, but from what I remember she lives in an oppressive society. She must keep her telepathy secret. Somehow the girl and boy found out about each other and start communicating with each other telepathically across the galaxy.
And the alien voice in his head is not so nice after all...The girl must travel in a spaceship with an older man who finds out about her telepathy so they can rescue the boy from the alien who is trying to mind control him. That's the gist of the story.