It's far easier to earn money from writing non-fiction than from writing fiction. There's a bigger need for articles than for short fiction: just think of all those newspapers and magazines with pages to fill every day, week and month. And when it comes to selling a book non-fiction is usually the easier route to take, for one very simple reason.
Unless you are very well-known, and your previous novels have made exceptional sales, you’ll have to complete your entire novel before you try to sell it. It's difficult for anyone to judge how well a whole novel will turn out with only a few chapters and an outline to go by, and so much depends on the writer's talent: a story can be difficult to grab hold of and control, especially over the length of a whole novel.
With non-fiction it's more usual to sell the book based on three chapters and a proposal, even if you’ve not published much before. The proposal should contain an outline of the book, perhaps given chapter by chapter; a summary of available markets and any competition; and some information about why the author is the best person available to write the book.
Writing a proposal forces the writer to ensure that there is a market for their book, and to pinpoint exactly what that market is. This provides the publishers with a clear understanding of exactly what they’re signing up, and so avoids a lot of the uncertainty that is involved with commissioning novels before they’re finished.
Although it will take some time to write a good proposal it is far less demanding on a writer's time than a whole novel: which means that less will be lost if the book doesn't sell. And proposals are far easier to write well than you might imagine, especially if you buy yourself a copy of Susan Page’s brilliantly useful How to Get Published and Make a Lot of Money. It contains step-by-step instructions and the exact proposal which got Page a contract for the book.