Edwin Abbot's book, Flatland, is out of copyright since it was published in 1884. So I wondered if the text could be found online for free.
It can. Here. You have to scroll past a long list of disclaimers to get there.
The ASCII representations of the diagrams are not great, but the text is there, including his (possibly overdone) discussion of life as a Flatlander. It's not a long book, though.
Personally, I don't like reading online for long periods. I prefer the real book, not least because some of the diagrams are necessary, and they're much better on paper. For those who can stand to stare at a screen, the book's now free.