I once asked for a list of books that Ravi Zacharias recommended. I was sent a list of 30 books; three of which were by George MacDonald.
What I really appreciate about MacDonald's books is his honesty.
Thomas Wingfold, Curate is the story of a minister, who like many Ministers in the 19th century viewed what he did as a profession. After being challenged by an atheist he decides to take a closer look at what he really believes. (It's the first in a trilogy.) The first 5 or 6 chapters are pretty slow, but they are a necessary introduction to his struggle.
Every minister would do well to read that book.
The second book in the trilogy is one of the books Ravi recommended, Paul Faber, Surgeon. Here is a sermon from that book about hypocrisy. (It is tempting to share an emotionally gripping chapter about two lovers , but I won't because it would ruin the story. I found this book more interesting than Thomas Wingfold and it's not as long. After reading this it's not hard to see why he was C.S. Lewis' favourite author. There's a beauty in his writing that I have not seen anywhere else.)