My favorite classic novel is the Hunchback of Notre Dame. The original version is very boring at times, when the Victorian building descriptions get out of hand, but 5th grade me fell in love with the plot. I read the abridged version, without the 3 page long rants about gargoyles.
I was a very naive child and I had never read a book without a happy ending before. (SPOILER: THIS BOOK DOES NOT END HAPPY). I felt betrayed by the author, Victor Hugo, and after finishing the novel, fell into a state of rage. This subsequently ended with depression, before nihilism set in, and my emotional spectrum ranged from "stale bread" to "cynical doomsday theorist". However, the book taught me that sometimes, happy endings don't happen. Life doesn't work that way. Whether or not you're "okay" is up to you. That realization was a very important part of my character development, and I have this book to thank for that.