I've benefitted a lot from some of the old threads here about the 35665A and other DSA's and thought I would share something back. I've put together a page with some notes about this instrument, including a list of what I think is all of the relevant documentation.
I was able to find scans of many of the documents online and have collected them here. A couple of documents (in particular for Instrument BASIC) I was able to source hard copies of and scan. I was also lucky enough to get a copy of the SDF Utilities and Example Programs disks, which I've posted images of.
I think all I'm missing is the HP-IB Quick Reference and the Service Manual (which I'd very much like to have, if you can point me to a scan or hard-copy for sale).
Anyway, here's what I've got so far. I hope it's helpful!
https://heartfx.net/hp-35665a.html