Ok, just found an existing copy on the internet from http://worrydream.com/refs/Griswold-TheSnobolProgrammingLanguage.pdf - seems it may be reprinted now.
I repeated the experiment with giving just the URL to the assistant, but it was less helpful than when I directly uploaded it:
It pretty much ignores the whole text of the book itself, which is probably because this version of the text is not OCRed. So that's maybe another suggestion: If substantial parts of a PDF miss OCR, give a warning :-)
But as I just rechecked my copy of the book and it also contains rights to redistribute it, I'll just attach it here:
I tried to repeat my original session with my OCRed copy. It was more intelligent this time, most probably cause I switched the default in Kagi's settings from "Last used" assistant to "Expert" assistant and so did not fall into the trap again, that uploads currently reset the used model to another one (my original experiment was most probably running in "Research/Fast" as it reset during upload)
It again was not able to provide the references from the book, though, so unsure whether it has even seen them, or whether it did see them and it's just the limitations of LLM in general causing the problem.
It also still exhibits the problem, where it will totally focus on the ads on the last page for the initial summary and only if told that that's not what the book is about, it will provide a proper summary.