Between fiction projects I decided to write a short non fiction book. And you can do the same about something you know and love. It took me about two weeks to write and organize the photos I had. I had it formatted on fiverr for $30 (for info on how to deal with fiverr, and what to expect, and how to avoid hassles, see my blog post on the subject), and now I'm selling ecopies on Amazon and hard copies alike. No, it doesn't sell like gangbusters. It is a small niche market, for a carburetor that was last made in 1979. But it has become my 'little train that could.' It just keeps trucking.
Every month I make about $20 from royalties. It sold a copy in France too. For me it was an easy way to build up my title list, and I have others planned.
Pick a subject you know a lot about, and plan a small non fiction book.