The Narrative of Arthur Gordon Pym of Nantucket by Edgar Allan Poe. He wrote a hollow Earth novel, decided it was silly, and never finished it. It was eventually released with a very abrupt and obviously incomplete ending.

The book should have been revisited. Some thought would have provided an answer to his plot issues and frustration. I don’t know where he was going with the ending, but he must have had some ideas. A sequel could have picked up where the book left off and explained where he was going.

I have never been more frustrated by a book. This could have been a classic with more effort and commitment.