A whole trove of links to scans of *old* maths textbooks, going back to Robert Recorde in the 16th century
https://www.resourceaholic.com/p/digitised-antique-maths-textbooks.html
The collator writes
"I have mainly listed algebra and arithmetic books (rather than geometry books) because that's my area of interest."
That page linked to this one:
https://www.forgottenbooks.com/en/Mathematics
which has a whole bunch of even more interesting books! But it seems there's a kind of membership model, or else some kind of restrictions on unregistered page views.
@highergeometer @11011110 Here is the page of Recorde's book “The Whetstone of Witte” where he invented the modern equals sign = : https://archive.org/details/TheWhetstoneOfWitte/page/n237/mode/2up
(I think the same book may also have introduced for the first time the + and - signs, but I am not sure.)
It is also noteworthy for the word “zenzizenzizenzike” which is the word from the OED that contains the most z's and which means the eighth of a number.
@mjd @highergeometer I think you're missing the word "power": zenzizenzizenzike means
@11011110 @highergeometer I meant to write “eighth root”.