I'll Always Love You by Hans Wilhelm (1985)
I first read it at maybe 6(?) It's the first book that ever made me cry and feel such complicated emotions.
Even thinking about it now, I'm starting to feel emotional. I would run into a burning house for that book.
If it's a finished series I always complete the series.
Common Sense by Thomas Paine (1776).
It was the document that explains the evils of monarchy and arguing that the British government is a bully and it's stupid for an island to try to rule a continent.
I read it 10 years ago and remember posting on Facebook "Is it weird that I now have a crush on a guy who's been dead for over 200 years?"
By originally date published, oldest to newest