Here's an impressive inventory of Books I've Never Finished:
I also once started a "Skeptic's Guide to Las Vegas." I had a real literary agent, and the project was considered by some major publishers. They liked the writing but came to this conclusion: Who would want to buy a book that disses the place you plan to visit?
I also wrote several chapters in a practical guide to divorce, in partnership with a Family Court insider, but the project died when my co-author lost interest.