We haven't been able to get old information from IBM; either they discarded it or don't make it available. There are lots of old manuals on bitsavers, which is a treasure for historical computer research. It is worrisome how much preservation depends on the efforts of a single person.

https://bitsavers.org/pdf/ibm/

One of my hobbies is writing period-correct stuff for s/370 and MVS, and, yep, bitsavers is where it's at. Weird thought that if that person didn't go through the effort, the information would be near-unobtainable.

This might be a bit morbid but consider contacting estate sales agents in the Poughkeepsie and Rochester areas? The last generation of hard core mainframe employees are all nearing retirement or have retired and as they and their families deal with the detritus of their careers may just be throwing away stuff. A lot of the print books ceased mid 1990s, in favor of BookManager files and eventually PDFs.