Where the Red Fern Grows, Wilson Rawls
Look, as far as we’re concerned, this book is not appropriate for children. Your humble author read it in the third grade, when the teachers realized I was bored stiff with the picture books they were prescribing to the rest of the class. I fell out of my chair crying. Spoiler alert, but at the end, the one dog dies, and then the other dog dies of sadness. That is just the worst thing I had ever heard. It still might be.