The thing that I have noticed is that, with a few notable exceptions like Robin Phillips' projects, most library music is a few years behind the popular styles and trends. So the production year is not really a good indicator of what sort of music is going to be contained within a lot of the time.
I don't think this is true of library music in the late 60s to late 70s, even into early 80s. It may be true that library styles were not in line with whatever was the most "cutting edge" at the time, but library music of these periods was superb in capturing the popular styles and ambient sounds that would've been familiar to everyone at the time -- library actually defined the sounds of the time, being in the background to most every film, radio and TV commercial, classroom documentary, cartoon, soap opera, sitcom, etc.. It permeated every aspect of our lives. I love history and I'm nostalgic, and also getting up in age, so for me this historical aspect of library music is why it was always so appealing. In my opinion, if you want to capture the sound, the mood of a particular year or set of years, there's nothing better than library music.
