Basically, the number of titles for all books before 1980 is only barely greater than the number of titles in the 24 years since 1980.
Figure 4 shows numbers of holdings in the entire OCLC catalog:
Pre-1980: 330,291,378ARL libraries aren't as top-heavy in the 1980-2004 level as are other OCLC libraries, which makes sense. But what this does tell me is that libraries are facing a huge increase in volumes that they must manage, and it must be because there is a lot more being published. Wikipedia reports these figures for the number of titles published per year in the UK and US. Presumably this is book titles only:
Note the near doubling in the UK, and near trebling in the US. This may not be news, but I have never before seen figures that confirmed my general feeling that it's all speeding up.