When I was a teenager, I read a sci fi novel that I have since lost and cannot remember the title or the author. Published in the late 80's or early 90's I believe. I would like to find it again, so wonder if anybody might know the book I am looking for.

The basic storyline is that some scientist types go undercover on a foreign world to study the people. They have a directive similar to the idea of the Star Trek Prime Directive where they are under no circumstances to interfere with the course of events on the planet. The main character becomes quite enamoured of the Queen of the main population who is suffering from an illness (something akin to, or may be exactly, cancer?) and decides to betray the directive and give her medical help. A team returns much later (I think decades or maybe even hundredes of years later?) to find that the Queen is still alive and has essentially been made immortal by the medical treatment and, being a progressive thinker, has seriously advanced the society but, of course, it has wrought negative consequences as well.... and the story unfurls from there. I seem to recall the mass paperback edition that I had showed an investigator on the front cover with a helmet which covered the entire head and the front was a reflective/mirror plate.

If anyone knows which book I speak of, I'd appreciate the help! Many thanks.

noninterference by harry turtledove sounds like the book you are looking for

http://www.paperbackswap.com/Noninterference-Harry-Turtledove/book/0345343387/

i just finished reading it and it's what you described

This is exactly the book I was looking! Many thanks!!