EZ-GO by Bruce Wilcox is available as pdf free online: (PDF) EZ-GO Oriental Strategy in a Nutshell | Samuel Dos Santos - Academia.edu
I think it is an underrated Go book since he is very good at translating into general rules and patterns how the best Go players normally played before AlphaGo.
I also find his interactive Go Dojo (interactive “books” where he further develops the principles presented in EZ-GO) to be useful despite the fact that the graphics are extremely outdated: