Is ECO, Encyclopedia of Chess Openings still considered the best printed reference work for all the chess openings, as it was back in the '80s?
There does not seem to be a consensus on this, but here is a relevant discussion:
For printed opening material (as you requested) I prefer ECO with Chess Informant updates for new opening ideas and analysis: http://www.chessinformant.org/about-chess-informant/
IMO, this is an area that needs work. I developed a small computer program to check for duplicates in openings and found that most opening categorizations contain duplicate positions. For example, Schiller's opening list has duplicate openings. The same positions arise through different move orders but are named differently. ECO is far too general.