Most of us are hard at work on the next great American novel. But don't forget about the non-fiction market. More non-fiction titles are printed every year than fiction---by a long shot. There are some definite upsides to writing within this genre.
1. You aren't required to have a finished book to sell non-fiction. Most non-fiction is sold on proposal. A proposal is a formatted document that basically outlines the book you plan to write, the marketing niche it will fill, and a list of credentials which qualify you to write it. There are some great books and websites that can teach you how to craft a non-fiction proposal.
2. Non-fiction writers often have very long and profitable careers. It's often easier to sell non-fiction. And it's often easier to get an agent.
3. You aren't required to create worlds, characters and conflicts. Your basic storyline is already there. If you get writer's block, non-fiction might be an avenue you should explore.
4. There are more markets for non-fiction, and more ways to break into the business. Most magazines have stopped printing short fiction. However, they're always looking for freelance non-fiction articles. If you get a good publishing history in magazines, it's often easier to sell you non-fiction book because you've already proven several things: you write well, you can meet deadlines, and you are capable of meshing facts with your own style.
If writing a novel is your dream, don't give up. Keep writing. But don't forget about other opportunities that might give you a chance to improve your craft, make a little money and see your name in print. It might be a stepping stone to selling that novel.