First published in 1872, THE PRINCESS AND THE GOBLIN, by George MacDonald, is the oldest middle grade I've recommend on my blog. I adored it as a child and pulled my beat-up copy out for another read a few weeks ago. It's one of those stories that feels like an old friend. The cover that I found on Goodreads isn't the one I remember, but mine's been missing its cover for so long that I don't remember it well.
C. S. Lewis and J. R. R. Tolkien admired MacDonald's work, especially his children's stories, and credited him with influencing their writing. Lewis believed MacDonald to be the greatest of myth-makers and said that his stories shock readers "more fully awake than we are for most of our lives."
I've read and enjoyed more of MacDonald's work as an adult, but THE PRINCESS AND THE GOBLIN remains my favorite. It's the story of a princess who the adults try to keep safely locked away from the goblins that live in the mountains and a miner boy who uncovers a plot to kidnap her. The two children don't always agree with each other. In fact, the authenticity of their voices is one of the story's most charming assets. But they are there for each other when it matters.
If you'd like to read about more Marvelous Middle Grades, the following people would love to oblige you:
Shannon Whitney Messenger (our founding mother)