I've recently come across https://ia800104.us.archive.org/20/items/FoundationsOfMechanicalAccuracy/Foundations_of_Mechanical_Accuracy.pdf again, and it made me think.
There must be hundreds of "HN-interesting" books that are real classics in their field on archive.org.
Another example would be Halmos' "Finite-dimensional Vector Spaces".
Unfortunately, I think there is no ready-made collection for "standard works" or textbooks.
Have you bookmarked such works? And let's limit it to archive.org, if only so that the thread doesn't become a random book recommendation thing.