I really feel the sad bits in the books that are mostly happy. Haunted by a particular scene is a good way to describe it @Jocasta Braithe. Shadow of the Wind had some terribly sad moments, but it wasn’t constant in that book.
I don't know that this book is the saddest, but it was the first book I was ever really emotionally invested in as a young adult and I sobbed. It's called Reunion by Sharon Sala. It was just supposed to be a cheesy romance novel, but nope. The end killed me.
The only other times that I've cried that hard reading a book since then was Harry Potter because I was invested and The Name of the Wind by Patrick Rothfuss, there toward the beginning when it's talking about Kvothe's parents.
Wesley the Owl by Stacey O'Brien. The author is a biologist and wildlife rescuer and this book is about her raising an injured barn owl she's named Wesley. It's not inherently sad for the entire book but its hard reading about her medical struggles and includes animal death. I bawl my eyes out every time I read this book
Magic's Pawn (whatever the last book in the trilogy was called) by Mercedes Lackey had me sobbing on and off for days. The buildup to the end is heart wrenching and the epilogue had me 100% ugly crying
Just had a moment reading through this thread, taking notes on which books I might want to add to my "to read" list based on how much of a mess they'll make out of me and I had to ask "why do I do this to myself?" Sobbing mess? Sign me up!