Yeah—I was interested to see the huge differences between eighteenth-, nineteenth- and early twentieth-century novels especially. (I might look at some Victorian novels at some point, too). It's easy to think 'old = formal = no contractions', and in some cases that seems to be true, but the full picture is much more complicated!

Out of curiosity, I had a look at my own fic. Up in the Grey Hills has two do nots and no don'ts—so the sample size is small but I'm doing all right so far, and if I use do not once more I should use don't once to balance. It's certainly something I'll keep in mind in future writing!