The Fellowship Of The Ring by J.R.R. Tolkien (1954)
The Fellowship of The Ring adds darkness and drama into the world of The Hobbit. Where Bilbo’s adventure felt like a small event in a much larger story, Frodo’s journey concerns the entire world. The stakes are much higher and as a result the themes are intensified.
I guess I don't have much to say that hasn't already been said.
It is fun to read these books and see where everything else that came after came from. “Oh, there’s Harry Potter, and there’s Dungeons and Dragons, and there’s Game of Thrones. Like, wow. They really just ripped stuff right out of this book and rolled with it, huh?”