Keyword: “Slade”. May be name of character, part of title, even place name.
I read the book in the mid to late 60’s. It was a Western, not a children’s book. A Pony Express rider and a teamster hate each other for reason unremembered (by me), which leads to a life-long feud. Their lives are somehow inextricably entwined, and they keep encountering each other. At the end, the teamster dies. I believe he may have been killed while saving the Pony Express rider. The Pony Express rider is distraught, and realizes his life is no longer as full as it once was, and he feels sad and empty.