The above linked article tells about how to treat the Little Free Libraries in your area. Take one leave one is the most important one, but this article includes little things like not donating or selling books that you’ve found. Tidy up, including making the titles face out and take out any garbage that someone else might have left.
I haven’t seen too many more libraries pop up, but when I do, I make sure to visit. I will keep these etiquette rules in mind.