My favorite(s) would have to be the Griffin and Sabine trilogy by Nick Bantock. I initially read it back when I was in middle school, and it was the one set of books that my teacher wouldn't let us purchase off of her at the end of the school year. Each book is fairly short and you read it in the form of post cards and letters that you remove from envelopes, and it details the correspondance between the two persons that seemingly can't meet each other in real life (sorta like Lake House or Il Mare). I wont spoil it, but I felt the frustration and sadness in the protagonist throughout the read. Fast forward to my college years and I remembered about it and immediately purchased the set at a bookstore and reread it and even then I teared up quite a bit.

Another would be Hermann Hesse's Siddhartha which I've actually begun rereading in German to practice the language. And finally my guilty pleasure was reading the Robotech series, which landed me in quite a bit of trouble during middle school since that's all I did in class.