There have been mail order book clubs around for a long time, but the one I am thinking of was called Encounters, and it was in operation from the late 1970s--no idea if and when it folded.
Like many at the time, it was advertised in a Sunday supplement, and you got, I think, 3 or 4 books for a ridiculously small amount--4 books for £1 my memory is telling me. You had to agree to buy a minimum of 3 or 4 books in the first year and there was a club magazine each month with new book releases.
The club featured books on the paranormal, mysteries, UFOs etc. I am fairly certain that my 4 books were:
World Atlas of Mysteries by Francis Hitching
Photographs of the Unknown by Bob Rickard
Secrets of Space by ?
Secret of Atlantis by Otto Much
I believe I only stayed a member for the one mandatory year, probably 1979, and cannot remember the books I was obliged to by in that time. All were quite hefty hardbacks. Secrets of Space was very good and introduced me to Einstein's theories of relativity, with clear and easily-understood explanations.