In a sense some may find this more of a band-aid than anything else, but I've found 95% of the book I want to read are available through libgen[0] and close to 99% of papers through sci-hub [1]. Sure enough, the book referenced in the great-great grandparent to your post is on libgen and I now have a PDF.

It really is unfortunate though, that there are a great many books one can only get a physical copy of for many times what they originally cost.

[0] https://en.wikipedia.org/wiki/Library_Genesis [1] https://en.wikipedia.org/wiki/Sci-Hub