1 vote for A High Wind in Jamaica.
I don't know which to choose for my other vote!
Here are some interesting articles on the history/evolution of young adult fiction. I like the timeline in the Real Simple link.
https://www.oxford-royale.co.uk/arti...t-fiction.html
https://www.realsimple.com/work-life...-adult-fiction
http://www.cnn.com/2013/10/15/living...ion/index.html