A Clockwork Orange by Anthony Burgess. Because it is a traumatic masterpiece. I was 15 when my mother gave that book to me. And meeting Alex who was my peer, provided a very different perspective of life and of something called 'the system' I kept hearing from adults often, a very different kind of dystopia at that age. I actually think this book has gained new kind of importance in the last 10 year, esp. in relationship to The Catcher in the Rye. (A completely different discussion, not suitable for this forum.)
After that when I have read The Catcher in the Rye in late teen years, I didn't get why it was labelled so dangerous in the English speaking world. They kept squeezing the name of the book or Salinger's name into everywhere possible in movies. What do we know about Salinger? Alltogether! He was a very private person! I assure you, a lot of people out of the Western culture who has no idea of the book, remembers that a man called Salinger lived somewhere and he was a very private person. Poor man. Also I always thought all Holden needed was just a hug. He probably looks like a pitiful character to young generations today.
Yeeears later, as an adult when I saw Kubrick's masterpiece which was banned before, I realised I haven't forgot anything about it and it was there with all its abhorrent magnificence. Probably Kubrick helped a lot as he does.
I don't need to read these two books again for different reasons. None of it is because I think they are bad. They are just, is.