To be honest, I don't expect anyone to know this book but I really like to know what it was called. I only got to read a few chapters but it got lost when we moved and I was never able to find it. It was probably left behind.
What I know about it..
-The author was most likely russian as it took place in russia and the main characters had russian names
-written before 2000, most likely before 1980 although I'm not sure.
-there was at least 2 books or stories in the same universe, they were both in the single book but had different main characters
-the first story was about an alien invasion where aliens took over people with some device that could insert or pull consciousness out of people or insert it (each person could hold up to 3, and the aliens took control over adults while younger than 18 took control of all). The consciousness, when extracted was a little marble sized ball
-the main characters were 2 teens and an older female doctor.. could have been more.
-the second book was about 2 different teens going to alien planet and taking alien bodies, probably by accident. I never read it so don't know much more though.
Not much to go on, I know.. I only know that book was translated into English, so the author couldn't have been that obscure.. still, it was a Russian name and hard to remember.