Anything before 1923 if outside of copyright and free for public use. Same goes for anything 70 years after the authors death.
For rare and out of print books I personally think that sharing a page or even a useful chapter online is no problem as access to information overrules Whatever other issues you might have. Of course if the author is alive and trying to sell books that’s a different story but since this thread is referencing very old things I don’t think we have to worry about that.